I do not hv the mentioned modem/router but these are all Networking / Virtual LAN terminology or protocols :-
VLAN tagging = this port hv VLAN numbers tagging on it, can only be used with another switch/router that support VLAN protocol/802.1q ( original DIR-615 )
IP Bridge = traffic is pure IP
PPPOE bridge = traffic is encapsulated with PPP Over Ethernet ( use mainly for authentications )
Vlan 500 = used by Unifi for Internet traffic
Vlan 600 = used by IPTV
What you need to get it working without the DIR-615 is this settings :-
Incoming line is a VLAN trunking with both vlan500 & vlan600 on the same port...
You will need to split them to different ports and remove the VLAN tagging...
- 1 separate network port for Internet with VLAN 500
- 1 separate network port for IPTV with VLAN 600
*** remove ALL VLAN tagging on both the above ports