Regarding Mobile antenna selection, there are a couple of antennas offered on the Teltonika website. It will vary on which type of solution are you trying to work with let say if you want to put the router outdoors you need to have a casing that is IP67 rated, the same thing with the antennas.
There are some 3rd party vendors also that is offering antenna for different situations.
Antenna options | Teltonika Networks (teltonika-networks.com)
Outdoor antennas and enclosures for Teltonika routers - QuWireless
Home - Poynting Tech - Antenna Solutions, South Africa, USA & Europe
There more available online :)
I hope it helps.