Hello,
Please take note that during firmware update process computer completely looses connection to the router. The process percentages which you see in the browser while device is being updated is purely shown by the browser itself (router's firmware update menu window, which browser loads before the update process, has information what % value to shown after certain amount of time has passed after the start of the update process).
If your browser stops "counting up" the percentage values, this is related with the browser itself, not the router. You can close this page at any time after starting the update process, since the update is happening inside the router and web browser's window is not interacting/interfering with it in any way.
Simply do not power device off while device is being updated (LAN LEDs are flashing "back and forth").
After initiating firmware update, wait few minutes and then simply try to access router's "LAN IP (default 192.168.1.1)" from same or separate web browsers window. Router should be reachable.