Back in the old days, we use to call that a multi homed computer. When routers were still very expensive you could take a PC with twin NIC’s, hook one to the internet, the other to the internal switch. You address your internal network to use the PC as your gateway then have the multi home computer route the packets out the secondary connection to the internet. It has been a long, long time since I did that. You could google multi home computer or multi home network or even PC as router and probably get the config.