There's a few points in here, but as a first check is anything reported on the service status checker for where you have problems? Use "check status" on https://coverage.ee.co.uk/coverage/ee
Do other EE users have similar issues in the same locations? Out of interest, do you have any specific example locations?
You mention changing to another provider, but also that you've had coverage in areas that no-one else did. Trial a SIM from another network provider to ensure it gives you the coverage & service levels where you need them. EE's 4G coverage footprint does continue to be industry-leading in remote locations especially - but there will always be local differences.
