Problem with modem

I’m new at freepbx.
I have a problem with my internet connection caused by the “freepbx” server computer.
If I need to disconnect the modem for any reason, and start it again, the modem doesn’t register to the internet. This is caused by the “freepbx” server computer, because if the computer is disconnected from the network, the modem registers normally.
When the modem is registered I can reconnect the “freepbx” server computer and all the network works fine.
I hope you can help me.

You did not give enough information to comment on your post let alone offer suggestions.

Please elaborate.

What information is needed?

As a wild guess, check how many DHCP servers exist in your network, and what they’re all doing.
But, as noted above, you’ll need to give much more details.