you are talking about other things, at least I do understand it in this way. We were talking about improving the WiFi-Reception IN a VAN from the RUT with metal walls and ceiling which is not optimal if you are working internally and the antennas are ALL outside of the box(Van). Nothing more! Sure, that the puck transfers the received signal to the RUT and what now? If it has to go to your WiFi devices, it goes back to the antennes in the puck. Or do you think that there is a mysterios secret WiFi antenne somewhere in the RUT black-box? It's not!
Your 3rd party credentials problem is the well-known "captive portal" problem of all these teltonika devices. Some people have succes in entering these portals by manipulating DNS-IP or MAC-Address, most others like me, don't have success, because this portals are working in different ways, there is no standard.
This is the answer from a teltonika moderator to your problem from oct. 2021:"Unfortunately, I can confirm that at this moment you cannot connect a router to a WiFi source with the captive portal as there is no configuration window to enter additional details such as landing page username and password. I will consult with our developers about the possible implementation of this feature."