No, it doesn't actually update all the in-between updates.
But usually, the latest firmware has all the previous fixes.
Depending on how far apart the router's firmware and that latest update you flash it to is, it may be wise/needed to perform a full reset afterward, followed by a minimal and manual configuration to secure the router and connect to your ISP. Make sure to NOT use a saved backup file either.
Have you tried rebooting the RT-AC5300 first, then try logging in again?
Have you tried pulling the power plug from the router and the AC wall plug, waiting a couple of minutes, then powering it up and try then (if the above reboot didn't work)?
Are you able to ssh into the router? You may be able to change the password there if you can.
You can use the same password, but to be sure you avoid similar situations like this in the future, be sure it is between 16-32 characters max. Also, be sure you are only using alphanumeric characters, without spaces, punctuation, special characters, or smiley faces contained within. Follow these rules for the username too (you do change the default username of 'admin', correct).
