We have a Dlink DWR 921 unit at work which we use at various events. Last time it was used was this time last year, where it performed flawlessly. However this time round, the unit is allowing connectivity but I cannot get into the admin interface.

I get prompted for username/password, click login, then get a page saying there was a problem and thats it. 

I've tried multiple browsers including rolling back to an older version of Firefox, but same issue, just cannot get past the login page. I have factory reset the unit to no avail. 

DLink DWR 921 LTE Router

Rev C

Any ideas.......