Not all 4G devices are the same. Phone's in particular get the higher category 4G transceivers first with other devices playing catch-up. Additionally, folks often retain a router longer than they do a phone. I note that my own Teltonika RutX11 only supports LTE Cat 6 while my phone is Cat 20. You can read about LTE categories here:
https://blog.linitx.com/what-is-lte-cat20-and-what-are-the-benefits/
On top of the category support, you also have band support. Phones again seem to get support for a wider range of bands, and the bands in use for 4G have been growing. If the local Tx that your phone connects to is on say B8 (900MHz), but the Teltonika does not support this band then it will connect to another signal, possibly much weaker. Detail on bands here:
https://en.wikipedia.org/wiki/List_of_LTE_networks_in_Europe
On top of this, external roof antennae offer different gains at different frequencies, but should in all cases remain better than the antenna's in a phone! I've had good results from a pair of these:
https://www.connextech.co.uk/lmo613...ultiband-mimo-outdoor-omni-antenna-1536-p.asp I currently use one for WiFi, but went with the pair for future-proofing reasons - future 4x4 MiMo for 5G and LTE cat 20.
The only solution is to upgrade your Teltonika. I am looking at updating to the 5G/Cat20 ready
RutX50, but they want almost £600 for it currently!!
Karen