I was a proud owner of 6 TP-Link WiFi routers. But now I have an issue with my new Archer C7.

I want to have truly mobile Internet connectivity and I want to use Archer C7 (AC1750) as a bridge to my main router (IP: 192.168.225.1) (a pocket WiFi device from China) which has Internet connection (via a SIM card). My laptop is connected to the bridging WiFi router via LAN cable. Below shows my network diagram.

I followed your bridging tutorial here but it doesn't work. However, the configuration in the tutorial does work when I replace the main router with my Samsung Galaxy S9 with Android 10 as a WiFi hotspot or with my home Internet router. 

Now here the funny thing. While I was troubleshooting, in the console UI, I decided to change my bridging Archer C7 operation mode from a router mode to an access point mode while still maintain the configuration I did following your tutorial and after the device auto rebooted, it works! I got Internet.

But recently I had to factory reset my C7 but after resetting and while trying the previous working configuration, it failed to work again. How come? I spent 5 hours but to no avail.

Under router mode, I can ping my C7 bridging router but I can't ping the main/root router. But the main/root router could detect that the bridging C7 router is connected to it.

In access point mode, the main router could also detect that the bridging C7 router is connected to it and it could assign an IP address to my laptop (IP: 192.168.225.23, default gateway: 192.168.225.1) where my laptop via LAN able with the C7 access point. But I can't ping the default gateway (.1) from my laptop and can't ping the access point (.2) too. 

Any one has any idea how to troubleshoot further?

Here're the screenshots of my configurations..