Try to open upgrade.mikrotik.com in a browser on your PC. It should take you to MikroTiks main page.
You should also be able to upgrade manually by download files.
https://wiki.mikrotik.com/wiki/Manual:U ... de_methods
If your device is connected on internett, I would not have been shocked if your router was hacked.
Running an more than 8 year old software (5.26) put you at very high risk.