You are right that the Device Connector is overkill (and discontinued) for your application. It would have worked, but wasn’t exactly it’s intended purpose.
In the Peplink ecosystem, your cheapest solution would probably be to put a AP One Rugged (APO-AC-RUG) on both ends and use “Wireless Mesh” in the AP tab of your B One settings page (or set up through Incontrol).
Basically, you configure a profile to apply a mesh using just the 2 Rugged AP’s (not the built in one in the B One or any others you have deployed in the house, barn or other buildings).
Use a wired connections from the B One to the Rugged in the house and to the Rugged that will be going in the barn so that they get set up, once settings are applied, move the barn one there, set them both up in as close to an unobstructed view as possible. In a window would be better than through a wall.
At 250 feet, this will likely work fine and you will have 3 lan ports for wired devices in the barn and a link that will operate at reduced wireless ac speeds due to distance and the antennas in use.
At that range, it also might not work at all. I wouldn’t make a guarantee without a site survey and testing it. Something as simple as tin on the barn walls could totally block it at that range. BUT, by choosing the Rugged model over some of the other choices in the lineup you can replace the removable omnidirectional antennas with directional alternatives. Also, if needed, the rugged is rated for -40 to +149F so if need be, you can stick it in a weatherproof enclosure, attach directional antennas to it and put it on your roof. You would need to power it with POE adapters if it’s installed like that though. Speeds will be faster with this setup than the omnidirectional antennas even if the omnidirectional antennas do work.
I won’t make antenna recommendations here but I will say that 250 feet is easily doable, 2 miles would be with parabolic dishes and 3 minutes aiming, so some directional panels with 2 antennas built in should do fine. Shouldn’t take long to find something if you do end up needing them. Try talking to your sales rep, they may have good suggestions.
Peplink does still have outdoor, omnidirectional access points available, AP Pro AX that would work (and probably a fair bit faster connections in this scenario) but the price is likely to deter you if all you want is some wifi, sensors and cameras in the barn.
All that said, wireless outdoor point to point bridges aren’t exactly an area where Peplink is focusing their product line. This usage scenario is a bit off of intended purpose of the rugged. You might be better off to use a matched pair of compact outdoor wifi bridge devices that are purpose built for this and install a AP One Rugged in the barn attached to that device to maintain your Peplink managed WIFI and network to the barn and any other outbuildings you decide to connect.
I am actually using both these setups right now for certain buildings, there really isn’t a right or a wrong answer.