Sorry for the delay I did follow your very good and easy to understand instructions.
Here is my detailed step by step
first SSH in
cd /etc

Then
mv ./AdGuardHome ./AdGuardHome.SAVE

This put the .SAVE on the original AdGurard directory.
Then
ln -s /mnt/usb_drv/AdGuardHome ./AdGuardHome

This put a symbolic link directory and all the files appeared in it.

At this point everything was working UNTIL I rebooted the router afterwards the AdGurard menu was showing under Applications, went into AdGuardHome noticed no statistics showing, clicked on settings and page was not found (http://192.168.8.1:3000/).
The USB and AdGurard directories were fine I also check the cd /etc still showing fine as above, I also checked in /usr/bin/AdGuardHome to make sure nothing had happened to the original symbolic link.

At this point I tried to reverse the first operations, deleted the symbolic link directory, removed the .SAVE at the end of the original AdGuardHome directory rebooted, at this point whatever I tried I could not get AdGurard to work, so I reloaded the firmware and settings and it worked fine with AdGurard back in cd /etc.

I tried this three times same result every time.

I did think the symbolic link was not pointing at the USB but that would not explain why I could not restore it back.