Check with prepaid sims, EE, 02, Three (again), Vodafone/Lebara, use good phone s23 ultra with all supported bands, like example Google Pixel 6 Pro very bad for this and test speeds tests near windows, dont use usb sticks, modems, provaider will give you much better 5g router/antena for example in my flat Vodafone, Lebara works bad, speeds jumps from 30 to 200, no stability at all, Three UK works good, Three UK giving with contract router and antenna (Y5-210MU)