ISP Blocking or suppressing iptv service (strange behavior)

Question

Curious George here… I hoping someone might have a clue for me. I have a Fiber PON connection from Telmex, and I suspect their router is blocking or suppressing my connection to the service provider. The stream buffers and never displays an error message. Sometimes, after about 10 minutes, the stream will start working. Using a VPN allows the service to function properly. However, here's where it gets strange. If I connect a second router with factory default settings to the ISP's PON/router, essentially creating a double NAT, I can connect to the second router, and the services work fine without the need for a VPN. This confuses me. I've tried changing the DNS on the ISP equipment to Google or Cloudflare, and I've also tried disabling the ISP's firewall. If they were blocking ports, it should also be blocked when using the second router, if I understand correctly. This is a question out of curiosity, as it doesn't affect my ability to use the service. But I'm intrigued by how they are able to do this, and why adding a second router overcomes the issue. Any insights would be greatly appreciated. Fyi, I have tested this at several locations that have this ISP. So it’s not unique to me.