Suggest you forget the mesh mode and use AP instead. Try:
ONT - (WAN) AC88U -
LAN IP > Hostname=router, Domain=lan, IP address=192.168.1.1 [Apply]
DHCP Server > Enable the DHCP server [Apply]
AC88U (LAN) - (WAN) AC68U - set in AP mode
LAN > Hostname=BasementAP, Domain Name=lan, Get IP...