Maybe you should check security and encryption type of your main router, which are recommended to be WPA/WPA2 Personal and AES. And you must choose the same type for your 854RE, or you will get the...

How far is the distance from 850RE to main router? And is there wall between them?

How you set up the SSID and password? Wifi Clone or manually?

The IP address of the clients connected to RE210 should be assigned by your router, not by RE210. That means the gateway of the clients should be your router's IP. So, maybe you can check the DHCP...

Maybe you should reinstall the software. I can create an account without any problem.

Yes, different models can work well together. Don't worry about it!