Can't be connected to internet and use LuCI at the same time

Recently I installed OpenWrt 18.06.4 on my DGN2200 v1.
I wanted to use it with my router/modem TP-Link Archer D7.
I changed LAN IPv4 to so that it doesn't conflict with Archer's config page.
Then I was able to open LuCI on new IP. However after I connected TP-Link to DGN2200 with ethernet cable my computer that was connected to DGN2200 got internet connection. At the same time LuCI stopped working. So that when my DGN2200 is connected only to my PC then LuCI works. But when I turn it on with both computer and Archer D7 connected then doesn't return any page.
How do I fix this?

The DGN2200 does not have a physical WAN port according to google images of this ADSL modem router.

Sounds like you have perhaps wired a LAN port from DGN2200 to a LAN port on your D7, and so you now have two conflicting DHCP servers operating on same physical LAN.

If your D7 is the internet facing modem router with IP address, and you want the DGN2200 to operate as a wireless access point, then change the DGN2200 to IP address for example and remember to disable the DHCP server on the DGN2200.

LuCI -> Network -> Interfaces -> LAN -> DHCP Server -> Disable DHCP for this interface.



The question is what do you want of adding the DGN2200 to your network.

If the purpose is to have extra LAN ports and/or WiFi, all on the same subnet as the main router, then what @bill888 said. You would want to connect the IP of the LAN interface to be on the same subnet as the main router's, just outside the D7's DHCP server pool (I would personally choose or as it's usual practice for static IPs pool to be at the beginning or the IP range), and turn the DGN2200 DHCP off to avoid conflict. note in this set up your DGN2200 will be just a switch/AP, it's not doing any routing and its firewall isn't utilized.

If on the other hand you want the DGN2200 to make a subnet and actually work like a router, with its clients separated from the D7 router and the firewall functioning, then you would need to utilize VLANs (if your device switch support them) to make on of the Ethernet ports as WAN port (probably make it a DHCP client), and the LAN interface on this case will be on a different subnet, like what you were trying to do.

More reading here about choosing between switch and router setup.