Question about TP-Link Archer MR400

In my flat, which is surrounded by concrete, the 4g signal is unfortunately quite poor - both with iPhone x and the mr400. outside it is a bit faster. Now I read that LTE antennas have to be authorised gm Bakom and you need permission from the telecom provider - does this also apply to mobile antennas like the Delock that you can connect to the Mr400? or do you have another idea? (I don't have cable internet as we could receive max 33mbit here, which seems a bit expensive at 55fr per month)y thanks for tips :)