@Gonz2 If you are going to go down that route, then would just Factory reset both devices, and once done, attach the smart wireless to the EE Router, either wired or wireless which ever way you want to do it, and see what happens. If it works, then add one device to the wireless and see that it works to your satisfaction and as you expect.
If all is good, then go change the SSID and Password again, see if both Router and Extender are happy after the reboot test connection again. If that is good, then all of the devices will reconnect as the SSID and Password for them are known.
Anything can happen, but can assure you that it is NOT the way that i have ever done it, and have been doing it for many many years.
I changed and setup my EE Router OOB not even connected to wireless day's ahead of the activation, set SSID and Password on the router to match the requirements, when activated by EE put router on the network, connected to EE and did zero else, every device connected as expected, had to set up the stupid app that is required and done.
