@shwelone 

If I understand you correctly, the ISP router is Router 1 and the AX55 is Router 2.  Router 1 is assigned the IP address of 192.168.100.1 and you want to assign the IP address 192.168.100.2 to the AX55 but the assignment is causing problems.

I would do the following:

1.  In Router 1, disable the DHCP server -OR- ensure its range does not include any static IP addresses, such as 192.168.100.2.  For example, 192.168.100.32 to 192.168.100.199 would be OK.  If nothing else is connected to Router 1, I would disable the DHCP server completely as it is not needed.

2. Connect the cable to the AX55 WAN port.  This will ensure the AX55 web interface shows an internet connection.  The LAN port will work but you may see a red X in the Network Map of the interface for the connection.

3. Set the static IP address in the AX55 as follows:
Internet Connection Type:  Static IP
IP Address:  192.168.100.2
Subnet Mask:  255.255.255.0
Default Gateway:  192.168.100.1
DNS Servers:  8.8.8.8 and 8.8.4.4  (these are the Google servers or you can use any others of your choosing)

4.  If you disabled the DHCP server in Router 1, then set the DHCP server in the AX55 to "On".  For the address pool, I would use something like 192.168.100.32 to 192.168.100.199 which will give you more than enough IP addresses while leaving some addresses free for static assignments.  If you left the DHCP server enabled in Router 1, then set the DHCP server in Router 2 to OFF.  It could be left in Auto but it is better to set it to Off.

At this point, you should not have any more address conflicts.

  @shwelone 

The steps in my prior post assumed your AX55 was in access point (AP) mode, not wireless router mode.  After checking my AX55 in AP mode, I see the VPN features are not available.  If you are using OpenVPN, then the AP mode can not be used.  I overlooked that point.

With your current configuration, everything should work OK.  Yes, in your case there are two subnets, one for each router.  It is normal for Router 2 devices to reach Router 1 devices.  This does not work in the opposite direction (Router 1 to Router 2) because the AX55 router firewall does not allow any Router 1 device to initiate a connection to the AX55 LAN..

Both of your routers are using NAT (Network Address Translation) which is a "one-to-many" way of connecting multiple devices to the internet using a single IP address.  Your Router 2 devices are using double NAT which, most of the time, works fine.  I am not a gamer but game devices sometimes don't work with double NAT.  If you have problems with your gaming, there are several things you can do.  The first would be to connect the gaming device to Router 1.  The second would be to open the ports on Router 2 that your gaming device requires. If you need further assistance with a gaming configuration, just start another thread in this forum and I'm sure someone will be able to help you.

Good luck!

  @jra11500 

I see. I will stick with this configuration and check if I encounter any problem in future then.

Thanks a lot for the help and explanation!

  @terziyski 

Today I tested the workaround with an AX55 in wireless router mode and connected it LAN-to-LAN with the main router (also an AX55).  In this configuration, I was unable to set up a SurfShark OpenVPN connection which does work on the main router.  When enabling the connection to the server, the GUI displays "Connecting" and after a few seconds "Disconnecting", followed by "Disconnected".  Afterwards, re-enabling the server connection on the main router worked perfectly meaning the configuration file is OK.

After reconfiguring the second AX55 (now WAN-to-LAN), the OpenVPN connection works fine even though it is working through double NAT.  It would appear that the router needs to use the WAN port for setting up a VPN client connection.