Can't access admin panel when connected behind ISP router?

Currently I have the Flint GL-AX1800 WAN port connected to a LAN port on the ISP Router. The ISP router ( is using DHCP and assigned to Flint, but I cannot reach the admin panel on that IP from my PC.

The only way I can reach the admin panel is by disconnecting the ethernet cable on my PC from the ISP router LAN and connecting it directly into the LAN port on Flint where I can then access it on both and default

What am I doing wrong?

On Flint UI pls open port 80 in firewall.

That worked, thank you. Marked as solution.