@Johan-Bygdea install external aerials.

However the actual problem is a very poor radio in the very old MR600 and you'll probably be better returning it and buying a better LTE modem or router. No one should be buying the MR600 today, it's underpowered and has never been supported well - possibly because they can't fix it's many problems.

@JerryAtrick Hi Jerry!

I have external antennas, free sight 700 meters to the transmitter antenna. 
Sorry to hear that it's a bad modem, brought it yesterday. 

2 years ago I tested against a 2015 era LG G4, in the same window and saw the phone consistently achieve 40mbps, best my MR600 EVER achieved was 21 and it got less than half that most of the time.

My 2020 Mi 9T hit's 20-60mps in the same place & gets 4G+, the MR600 cannot even detect the band needed for 4G+!

It's the radio, it's worse than a 2015 chipset, the MR600 was born obsolete. Replace it, worth doing to avoid all the other bugs TP-Link haven't bothered fixing before falling foul of them. It's a terrible modem with an appalling radio and the router is a bug ridden, underpowered joke.