I agree with
@MrSoul4470, but just in case, have you already tried a factory reset of your WiiM?
Today my new toy (AVR) arrived, and when I first tested Spotify Connect, but there was no sound. In conclusion, I solved it by system rebooting the AVR. The WiiM may be able to system reboot from the web using the IP address as well, but I don't know how to do it. So factory reset is an alternative.