@Dave89
If the 5G network is not stable, please try to modify the 5G wireless settings on the C6, such as wireless channel and channel width. It is suggested to run a Wi-Fi analyzer to find a clear channel, then change the channel width to 40MHz, reboot the router and reconnect to the wireless network, then confirm.
If the 5G signal never appears, and no device can find it anymore even after the factory reset, please try installing the beta firmware we provided in the below thread, then check again:
https://community.tp-link.com/en/home/forum/topic/271574