Welcome to EpoS Wizard!
Shouldn’t really need to change any settings on the router. Set up your own ‘internal’ network on a standalone switch, with all the tills and pc connected to this switch. Test network functionality on this network, any issues, it can be narrowed down to a possible cabling fault.
If you have no issues, best thing would be to put a seperate network card in the pc for external comms, link this to the original router, keeping all previous ip settings on this ‘external’ network
What PC? There is no PC.
Most modern EPoS setups don't have a PC in the system to control them. Also, using a PC with two NICs as a router is just asking for additional trouble.
With a compliant customer the way to do it would be to replace the ISP router with a Draytek unit and run completely separate networks for the EPoS kit and the guest network. Unfortunately most customers aren't willing to take the additional cost of doing things properly and just want to moan when they go wrong.