From the little I know on the matter, IPTV is tricky.
There is more than a single way the service can be provided by your ISP, you should first thing see if there is some (official or unofficial) documentation on how the service is provided.
Generally speaking if it is using a VLAN, it will only work if you configure the Mikrotik as a VLAN aware bridge, as you reported:
- Mikrotik as router (no VLANs, natted LAN and WAN) = internet but No IPTV
- Mikrotik as (dumb) bridge = IPTV but No internet
while likely you want: - Mikrotik as VLAN aware bridge (and natted LAN via VLAN) = IPTV on one port and internet on the other ones.
Very loosely the idea is that the packets coming from the ISP router are enclosed/encapsulated in this sort of protecting envelope, if the Mikrotik is setup as a bridge they pass through it as they are but don’t allow the internet packets to go through, if the Mikrotik is setup as router they break but allow the internet to packets, by making the Mikrotik a VLAN aware filtering bridge you allow these packets to go to a destination port interface untouched while the internet packets also can go through.
You can think at VLANs as a way to have different traffic pass through a single ethernet cable, the alternative to running multiple cables, but there is the need of something directing the traffic.
But there could be further complications, if the IPTV is multicast and/or IGMP snooping is required.
Before the cock crows today you will be pointed (one or more times) to this all-pervasive thread/tutorial on VLANs:
http://forum.mikrotik.com/t/using-routeros-to-vlan-your-network/126489/1
You can use the time before you get back home to study it, but it is not a direct answer to your question and it is rather difficult to go through, at least it could be useful to give you some familiiarity with the concepts and terminology.
Once you have some more accurate data, and you have the possibility to play with the settings, post again with your current configuration and the ISP data, I am sure that some of the members more ffamiliar with VLANs will be able to assist you.