Hello,
Teltonika itself does not manufacture antennas, though, I would suggest to look at the offerings from partner Poynting. One of the options could be this one. If its gain is not satisfactory, you could consider directional antenna solution.
The specific antenna depends on your requirements, whether you need WiFi only antenna or an all-in-one solution for mobile, Bluetooth and GPS as well, and whether the antenna is intended for internal/external use.
This link provides general guidelines, on how to select compatible antennas, though majority of the requirements will apply to most of available offerings.
Best regards,
