Hi,
It could be that you're ending up in another router WebUI or the operator redirects you somewhere.
If you're trying to reach Teltonika device through address 192.168.1.1, you can always connect to it to anything that doesn't redirect you to Telia login and then change the LAN IP of the device to something like 192.168.2.1 or 192.168.0.1..
But as I really don't know what is your topology of the network - I will not be able to help you detecting from where and how that Telia login page appears.
EB.