@Patdeco
Good day, and welcome to the community.
After checking the support ticket record, I wonder whether there is still more than one network name during the configuration Nest camera.
If yes, have you tried to leave the main Deco only, unplugged the satellite Deco X55s, then still turn off 5Ghz?
Then disconnect the WiFi network on your smartphone and connect to the same WiFi again to start the configuration once more.
If It failed again, please try to record a video clip of the configuration process(you could reply to the support ticket with the video clip) and I would like to forward your case to the senior engineers for further assistance.
Wait for your reply.
Best regards.