And I feel like it’s not a good idea to have a modem directly attached to the pc directly unless you’re using it as a router?

Yeah I feel like this is the issue. The modem/router would be firewalling between the networks hiding the PC behind it.

Also from the description, does OP have a router at all? Is their ISP somehow just allocating public IPs to everything? Do your IPs start with 192.168 or something else?

Someone has also just done 100 modems over a T3 line using Cisco gear: https://youtu.be/rOdGK6GVIVU

https://archive.md/mVUfw