Just thought I'd say I looked for an admin page and couldn't find it either. My firmware version on all 3 is 20181127.17.1
Up until now, I hadn't bothered trying to change the password. My login details were also Admin/Admin by default, never even noticed I always just assume password is username.

Side note my 3rd showed up yesterday and is now set up. So far it looks to be 31.7 TH, and by far my tightest cluster on frequency, 499 - 507 - 498 .
That may be due to the cooler garage environment.