How to fix network conflict?

Question

Computer networks

Given: a group of PCs in the local office network, and an MFP in the network. Subnet without Internet access. Each PC has a USB modem that provides access.
These two networks have constant conflicts, either the Internet is cut off, or access to the MFP is cut off on LAN. Nothing helps except restart.
Question: what could be the problem and what to do about it.

UPD: different subnets

Answer 1

Solution:
1. regedit from admin
> Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters > IPEnableRouter change the value from 0 to 1
2. when the modem is on without other connections. when the internet is running, cmd from the admin
> route print > look at the subnet given by the modem (on the route 0.0.0.0)
> route -p add [the subnet given by the modem, for example
192.168 . metric 20 (approximately smaller)
3. ibid
> route -p add [subnet where the LAN goes, we have 192.168.1.0] mask [corresponding to the subnet where the LAN goes]
[local gateway in this subnet, we have 192.168.1.200] metric 200 (about more)

Answer 2

Change subnet on PC or modems. to make them different

Answer 3

Are you sure that usb modems have LAN?
Do you have your own gsm gateway?
USB modems establish a vpn connection to the provider and their gateway is set by default, that is, if your route to LAN is not specified, everything will fly to the modem's gateway or to the provider's network that does not know about your network
Question how does this even work?