NR7101 How to connect & configure via ethernet over LAN?

I want to configure my NR7101 via ethernet connected to 'main router' from a  desktop PC connected to the same 'main router'.

When I connect the NR7101 to a PoE Switch or my main router with 192.168.1.1 of course there is a clash because the NR7101 is 192.168.1.1  by default. 

So I connected my phone to the NR7101 via its management Wifi, then changed NR7101 to 192.168.1.20, set DHCP to relay mode and turned off the IP4 firewall. 

Then if I connect the NR7101 ethernet cable into my PC like this I reach it from the PC:

PC[192.168.1.4]--->PoE Injector "LAN"
PoE Injector "PoE"--->NR7101[192.168.1.20]


But if I connect like this I cannot reach the NR7101:

PC[192.168.1.4]--->Existing router[192.168.1.1] port 2
Existing router[192.168.1.1]port 3--->PoE Injector "LAN"
PoE Injector "PoE"--->NR7101[192.168.1.20]

Both PC and device are on the same subnet.  Firewall off on both the existing router and NR7101.  The existing router does not see/list the NR7101 as connected.

How can I connect & configure via ethernet over LAN?