I would just learn how to utilize the IPTV settings that you showed above. However, you were asking about a bridge mode so I'm not sure if you want to do it this way but you could manually assign the IPTV device (MAC address) a manual IP from (under the LAN > DHCP Server settings > see below - assign the specific IPTV client and manual IP and then click apply) and next enable DMZ mode for that assigned LAN specific IP address in the Asus router settings under WAN > DMZ. This should fully expose the IPTV client to the WAN. Some clients such as VoIP devices that have their own DHCP server prefer this.
1) I am not familiar with your ISP. You will have to ask your ISP if the setup you are trying to do will work with their network because it looks like your other router is going to be a DHCP server. Some ISP's prefer or require that their equipment is detected and registered on their network for you to utilize their service.
If the other router (not Asus) was supplied by your ISP then you might have to put that router first with the ONT Ethernet cable plugged into the other router's red WAN port. It's DHCP server will be configured as 192.168.1.1 (or possibly 192.168.0.1). If that is the case then in the Asus router settings you should configure the Asus router's DHCP to a different starting IP address like 192.168.2.1 and connect the cable from the other router's LAN port to the Asus router WAN port. Then in the other router's settings select a "bridge mode" (if available) for the Asus routers IP address in the other router's settings OR you could put the Asus router's (MAC address and/or) IP address in the other router's DMZ.
2) If the other router is NOT an ISP supplied router and/or your ISP allows it then you could put the Asus router as receiving an Ethernet connection from the ONT as you show in your diagram. In that case you could connect the other router to the Asus router similar to the way you have in your diagram above except you will need to change the other router's starting DHCP address (for LAN) to a different address such as 192.168.2.1 (and the Asus router could remain as 192.168.1.1). Next you would need to manually assign the other router's IP address (WAN) which you show as 192.168.1.254 (and MAC address) and then put that same IP address (192.168.1.254) into the Asus router's DMZ as I described in my first post above.
3) Above are the ways I would normally configure the network topology. There might be another way to accomplish what you are trying to do so I encourage input from other forum members.
Note: The terms for "bridge mode" with wireless routers can be confusing. There is the wireless media bridge mode available for Asus routers as described here but that of course is not what you are trying to do.
