I believe a new local ISP may have incorrect configurations on their back end effecting my entire town.

So basically a new company came to town offering fiber internet for awesome prices, everyone is jumping on board since it's the first real competition we've ever had.

Myself and a few IT guys I know have noticed and reproduced several times a very odd issue that ONLY happens with this new ISP.

Any type of file sharing, media serving, any type of connections to a central server while the server is hosted on the new ISP will not allow other users of the same ISP to connect.

So for example:

Person A has Server A on internet hosted by new fiber company 5 People also using fiber company cannot connect to Server A 5 People not using fiber company CAN connect to server A

Person A has Server A on cable internet company All people including fiber company customers can now connect to server A

The issue appears to either be within the fiber company's Modem or with their infrastructure. The same routers were used with the same settings for all of our testing so the only variable is the fiber company's equipment or something on their backend.

The issue absolutely acts like a Nat translation issue from what I can tell however every technician at the ISP we have spoken with continues to blame our equipment even when only their modem in bridge mode directly connected to a computer was used.

Is there something else we can test to verify the issue is on their end? Like irrefutable proof? The issue doesn't hurt our group at all it's just an annoyance we noticed and we don't want our small town being plagued with another company screwing people over or not knowing what they're doing.