>Is there a possibility to configure two or more ports to IPTV services keeping only one port to internet
The reason one port is setup is usually that the ISP has configured the router to provide prioritised traffic to the single port they say to use on the router. Whether you can configure the same for other ports depends on what access they give you to the routers web configuration.
In the UK there are plenty of TV set top boxes that are independent of the ISP, so a lot will depend on which actual ISP you are talking about, am guessing you are abroad, as what you describe matches no UK ISP
Again the ONT are usually very heavily locked down, and the IPTV can be configured to be only available on a specific port. If the ONT out of the box supported more set top boxes then you would be expecting the ISP to be selling a multi-room solution to increase take-up of the service.