Had issues with Sunsynk inverter not connecting to new EE smart plus router after charging from BT and going full fibre. Took ages for me to identify the issue which was that the router is the problem. Changed router back to my BT smart hub 2 and inverter connected automatically. EE technical told me afterwards they are having a number of issues with these smart hub plus routers.

Had this issue with a friend's dongle,

Fix for his was the router was auto selecting channel 3 which is an overlapping channel on 2.4ghz, seems the dongle basic WiFi implementation didn't enjoy overlapping channel or auto, try change it from auto to manual and set channel to 6 or 11. When I did this it connected immediately