Can you connect with mobile phone to 5GHz tethered network?
As I understand, for your devices to be able to connect to such an isolated tethered AP on AM6B+, you must have a DHCP server to assign IP addresses to attached devices. I’m not sure that this possibility exists in CE.
When you get internet from any router, then IP addresses are provided by routers DHCP server. Try with assigning fixed IP addresses to your devices to skip otherwise needed DHCP server…