I had the same issue, i.e no access to 192.168.8.1, even after (successfully) uploading new firmware. I reached out to Gl-inet support who sent me this message:
Our engineers have released a special firmware for fixing the issue on your MT2500 router, do you mind giving it one last try?
Here is the link for downloading the new firmware from Dropbox:
https://www.dropbox.com/scl/fi/wul82z0a6iupmjbbd4muf/openwrt-mt2500-4.6.10-1118-1731927038.bin?rlkey=tq1uqt3fpl9fkbuic6wspymtj&e=1&st=1adligcp&dl=0
If the link above cannot work, please try this one:
Please try to flash this new firmware into the router via Uboot.
Here is the link to the steps again:
(removed because only two links allowed per post)
By the way, could you also send us the MAC address of this router?
It is printed on the router's label. Or you can send us a picture of the label and we will help you collect the MAC address.
And to my own surprise - it WORKED. Thank you to gl-inet support!