I was trying to access online demo routers as described on the page https://mikrotik.com/software but can't login. I'm using username "demo" and no password but getting "Authentication failed: invalid username or password." for https://demo.mt.lv/ and for demo2.mt.lv is Loading forewer.

Does it stil work? Please advise

http://demo2.mt.lv/ works for me

Demo2 does work, but asks me to change password.

If you like to try RouterOS, just download an ISO file and install it to an PC or VmWare. Or what many other does, download EVE-NG and install RoterOS there. (all free)
Then you can test and learn it.