First of all, I'm not sure about your current office network topology, here is the answer for you with the original router connected to the public network and the new router connected to the old router:
Connect the TL-WE840N to the Lan port of the original router through the Wan port.
Set up the Wan port of TL-WE840N, set up the Wan port IP of the router according to the original router (choose to dynamically obtain or statically specify according to the specific situation, which mainly depends on whether your old router has turned on the DHCP service), and fill in the DNS according to the public network information obtained.
IP conflict occurs because your IP uses the same network number, the solution can be this: access to the first level of routing using 192.168.1.X, access to the second level of the router using ?192.168.2.X.