Issues with VOD on TV Devices

Not sure how to explain it properly because english is not my main language

I installed the XR500 a month ago using my old modem/router as a bridge and everything works great - i got a  PC connected with LAN cable and plenty of wifi devices..

I also have a Digital TV converter  from a streaming company in my country and they work with Settelite for channels

but they also have a VOD service that requires an internet connection to work..

and for some reason when pluggin the XR500 to that TV converter(we have 3 of them) the VOD service not working

sometimes it just not loading pictures and sometimes you cant order movies and so on..

I got 2 TV tech guys here and both said its the Router and some "setting" block it from working properly because if i plug my old modem/router everything works fine.. - I told them it's seems wrong because internet is working flawlessy and if i plug a PC or any other devices they all work great so why they blame the XR500?

Anyway - b4 making my move and transfer to another company.. i wonder- are there any settings that can actually cause problems with the VOD service that i mentioned? anything worth changing just for the test?

thanks