I have the same issue and went with EE. I bought a Huawei B535 4G Router for less than £100 which has been superb. Get the one with the bunny rabbit antennas, makes a difference . I then added a directional outdoor aerial which improved stability and speed but I was in a difficult area for 4g. If you have a strong 4g signal then you dont need it.
(I got mine from the rainforest place and got a one month payg sim from all the suppliers and tried them all before settling on EE for us. I figured if none of them were viable I would just return the router
).
One other tip, move the router around the house. Putting it in the window where you can see the transmitter aerial "should" in my head be the best and fastest signal. That isnt always the case and you might find the other side of the room works better (soemthing to do with metal frames around window openings messing with the signal and also signal bouncing off walls in the corner of a room improving things.