OTD500 is antenna + router in a single box. There is nothing else needed. I just put it outdoors with a POE cable. Originally, I used it as a backup, but now I have an unlimited SIM, so I use it as a second internet connection.
Seems weird to cripple the product by not allowing me to (optionally) disable the internal antenna and instead use and tune an external antenna. And I suspect that is likely to make a difference when you are on the edge of coverage, but you know exactly where the relevant cell tower is, a few km away.
If you mean the standard routers (like the Rutx50), Teltonika itself sells external enclosures with antennas. https://www.teltonika-networks.com/products/accessories/ante...