Low mbps

Michael_3774232​ You might be in an area with poor coverage where you are far from Three's masts (or Vodafone if MOCN has been rolled out in your area), or indoors where the walls might be blocking the radio signals. Check the coverage here, and check the status of the network here. Ofcom also has a coverage map here, and there is a coverage map in the Speedtest.net app which is crowdsourced.

You can try switching to use only 4G Network Mode, as maybe your phone is holding onto a weak 5G connection. Otherwise, you might just have bad coverage with Three's network, so you might need to try another network under EE or O2.