I'm active in the classic car world, if you couldn't tell by the family cruiser in my profile pic - where security, theft, and tracking are frequent subjects of concern and discussion. There are many options that cross over to trailers.
I am not an expert, but I've spent many years absorbing feedback from the guys that have used all of these systems; and think I can give an overview.
Just a quick run-down of the general consensus for vehicles that can drive themselves away, and don't even need to be towed (generally - even though the easiest and often quickest way to steal a hot rod is with a flat-bed tow truck):
There are "active" measures and "passive" measures. I'll hit passive last. There's only one that's fairly 'universal'.
Recovery: As mentioned, cops don't have time to look for your property if it just "disappears". ...Unless it is EXTREMELY unique and identifiable - like a fluorescent pink teardrop trailer with a bright yellow banana on the side. Then they'll keep an eye out, while addressing more important matters (like the 14 year-old girl that was just found in an alley, beaten and raped).
They do, however, LOVE catching bad guys in possession of stolen property and/or commission of the crime. Having live tracking data turns a stolen vehicle (whether a '54 Ford Victoria or teardrop trailer) into an item worth pursuing.
Trackers:
Lo-Jack.
If you frequent areas where law enforcement is equipped with Lo-Jack equipment, it is the best option. It has the highest recovery rate, because it's more than just some GPS coordinates that show where the car
was somewhere between 90 seconds and 25 minutes ago. Their data is as close to live as it gets. Lo-Jack can also do close-proximity directional locating on the ground. ...And the places that are equipped for it are used to using the equipment. They love it.
Lo-Jack isn't everywhere.
Subscription-based trackers (other than Lo-Jack) get a lot of talk and a lot of trials. However, people that have used them - even if a vehicle was stolen and successfully recovered - frequently suggest the oft-overlooked alternative that's next. Recovery rates are getting lower every year, as most car thieves know to take the vehicle to a semi-secure location away from the theft, disconnect the battery, and look for a tracker that needs to be ripped out, before continuing on down the road; while more and more are using 'grey market' devices to just jam the GPS signal (so that the device can still transmit until its killed, but it doesn't know where it is).
A cell phone.
In today's world, one of the most affordable alternatives to subscription services is a pre-paid cellphone with an app that can send live tracking data to another phone number, or allow tracking on the internet. - Or even a security app that allows commands to be sent remotely to also enable the microphone, take photos, etc.
GPS jammers will render a phone obsolete, too. But I wouldn't expect one to be used for "low end" trailer theft.
Some guys in the car world treat them like hidden trackers. They hide the phone in a secure mount, with a charging cord attached so it won't unplug, with a dedicated backup battery that's also hidden somewhere near the phone; so the chance of recovery is extended if the primary vehicle battery is disconnected or drained. There's a fellow Nova guy that has a phone hidden behind the instrument cluster, with a peep hole through the clock. Aside from tracking the car, it takes a photo of the driver and sends that photo and GPS coordinates to the owner any time the car moves more than 300 feet from "home", or is already away from "home".
(He has a HUGE collection of pictures of himself driving his car.

)
Most guys, however, keep their cars locked in garages most of the time and don't want to be bothered to keep up with a hidden cellphone.
So, they just keep the phone on a charger in the house or garage, and toss it in the car before they drive away. Popular hiding spots are under spare tires, under seats, or even just under a floor mat or trunk carpet; aside from vehicle-specific nooks and crannies that work well (like the crack known as the "trunk drop-off" next to the fenders, or on top of the glove box, in '68-'79 Novas).
This gives potentially a few days worth of tracking for recovery.
The groups that I'm associated with represent about 1,800 cars, or so. We've had six (reported) vehicles with trackers stolen in the last two years. Four were recovered with cellphone tracking. One was recovered with a subscription service. The last one was chopped beyond belief before police got to the location (but the tracker did work).
Passive:
Chains. Beefy, expensive chains. And good locks.
Chains that can't be cut with hand tools, and take time to grind, are a good deterrent. Equally good locks are important.
None of the paranoid classic car guys that I associate with, that use high end chain to lock up our car(s), has had one stolen - even if it was broken into and someone was clearly looking to take it.
I didn't even bother insuring my tent trailer. I use a 20' section of 1/2" grade 100 lifting chain and a good (expensive!) lock, to lock up my trailers (plus double coupler locks). It cost me about $300 for the chain and lock, but I think the expense was worth it.
When there's a vehicle, another trailer, or tree to lock to, I just run the chain through the trailer's frame. Simple, easy, and quick. If there's nothing to lock it to, I run through both wheels with a loop around the axle. If someone tries to take it, they're going to have to drag it -- even if they were to put another set of wheels and tires on it, the locked wheels will still drag under the trailer.
Luckily, there are lighter versions available, because the 1/2" chain that I use is overkill, not for the faint of heart, and not to be used near fine finishes or nice wheels. That 20 foot section weights 93 lbs and is not kind to the surfaces that it touches. When used for my Nova, I simply go over the axle, inside the leaf springs. I don't want to mess up the trim rings or powder coating on the wheels.
If you consider using a beefy chain and good lock, just remember to keep that lock off of the ground and away from run-off, to help keep debris out of it. There's no point in paying through the teeth for a lock if it's just going to get packed with dirt and mud on its first use.
Lastly, insurance.
I have really good coverage on my Nova, with an agreed upon payout value. That helps with peace of mind when I take it places. I would hate to lose the car. But an insurance payout is better than nothing.
I use Hagerty. They claim to insure custom trailers. Perhaps I should check to see if a scratch-built teardrop qualifies. (I don't see why not. Scratch-built car haulers do.)