RB850X2 with STB

Hi everyone,

The router I am using now is RB850x2 and I have been facing a trouble that I cannot solve.

I made my home network small and simple in order to find the problem.
The ISP modem (ATU-R) connects to RB850’s ethernet port 1, a STB connects to RB850’s ethernet port 5, it’s a TV streaming service device given by the ISP, we call it MOD as I live in Taiwan.
PPPoE service is used to connect to the internet, and it is working properly.

When the ISP technician came to setup the internet and TV service, he told me the STB has to connect to the ATU-R directly or through switches only, a router before it would kill its connection.

So I did a test of making port 5’s masterport to “port 1”, everything works perfectly as the router now functions like a switch. But when I made port 5’s masterport to “none”, created a bridge “Bridge_WAN” and made port 1 & port 5 under Bridge_WAN, the STB would still work, but every 10-20 seconds I saw a broken/frozen texture.

Could someone please help me to fix the problem ? Really appreciate it !!

Ron