OP, the situation which you experienced is normal.
Before PCCW removed the all-in-one router, that router was doing the DHCP server job distributing internal IPs to your devices. Your AC66U was just an AP.
Right now the PCCW router is gone, so there's no DHCP server handling IP distribution anymore, that's why your PC couldn't get an IP.
You should manually setup IP on your PC to make it in the same subnet as your AC66U, then go into its setup page to enable router mode.