This is a new MT1300
I can get to the screen that prompts me for the Admin Password but can’t log in…
I’m using “admin” as the password but it says it’s not correct.
I tried resetting the modem but same results…
I could be wrong but I think the newer GL-MT1300 have a first time password on the bottom.
I do not work for and I am not directly associated with GL.iNet 
I misread you have already set it up
Try a private browser window
If that does not work you will need to try clearing browser cache
Tried… Still no luck
So you have gone through the initial setup(choose language and pick password and enter twice)?
Try with password blank hitting enter.
Wired or wireless connection?
Do you now the Firmware?
Screenshots of what you are looking at are helpful.
To stop wasting time I would reset and start new. “admin” password use might be prevented because of weak password check