Hi, Since the switch is in unmanaged mode there shouldn’t be anything that it is doing that is blocking traffic. Does the new router use the same IP/subnet as the old one? Are the PCs getting IP addresses through DHCP or are they set with static addresses. Can they communicate directly with each other through the switch? Are all of the connected ports getting link lights on the switch?
New router not recognizing devices on Dell PowerConnect 2716 switch