I would say YES when u connect TP-LINK TL-R470T+ / TP-LINK TL-R470T+ Load Balancer WAN1 to UniFi router LAN and WAN2 to Maxis Fiber router LAN. No PPPoE VLAN issue.
If u want TP-LINK TL-R470T+ / TP-LINK TL-R470T+ Load Balancer to dial PPPoE, please make sure it can support PPPoE VLAN tagging. If can't support PPPoE VLAN tagging, its NO.
Please correct me if I am wrong, thanks
Hardware TP-LINK TL-R470T+ Load Balancer, Set unifi or other ISP (P1 / Maxis ,Etc)