It's the first time you've posted about this issue, so rather than adding to someone else's thread, yours has been moved to a new post.
Changing your priceplan is just an account change, and shouldn't have any impact on your ability to receive coverage. Although if your phone doesn't have coverage, that will naturally impact your ability to make or receive calls, or send & receive text messages.
If you've not done so already, try toggling flight mode to ensure your phone hasn't "dropped off" available coverage.
Use the "check service status" option on https://ee.co.uk/help/mobile-coverage-checker in the first instance, to check for local service outages. The "report a problem" link should be used if no issues are reported but EE users continue to have issues.
If you're in the UK and have a WiFi connection, you can use WiFi-calling to keep using your phone as normal without needing network coverage.
