I've recently started having issues logging into a website (quicken signin page). It would hang when trying to access the password page. Checked all the settings on the TP-Link AX5400 Pro router and don't see anything that would cause this when it used to work fine I turned off the firewall, etc. Finally I switched to my old router and the issue goes away. I did a factory reset and still have the issue. So I'm trying to troubleshoot by downgrading from the newest firmware on my router (v1.6.1.3.1 Build 20240426). But when I try to perform the downgrade, I get the error "Backup file error".  I've tried with both older versions of the firmware downloaded from the support website and from multiple locations on my pc.This is connected by ethernet, so it's not a wireless issue. 

I'd appreciate any ideas to help. 

Thanks