RBR50: Can't change DNS settings (error msg that existing log-in can't contain non english words)

Hey Folks - first post here. I'm running an Orbi RBR50 with two sattelites. It's been running fine for a year but now I'm wanting to bypass my ISPs DNS because they keep having issues. My ISP requires a login name ("name@isp.net") and password which is in the Orbi admin page and, obviously working fine (as I have functioning internet). 

To change the DNS, I naviagate to my Internet Setup in the orbi admin web page, cruise down to DNS, click the button to enter my own settings, enter those, then click "Apply," and I get the message: 
"192.168.1.1 says:
Invalid Login name, can't accept non-english words"

My first thought was that I accidentally entered a character in the wrong field, so I logged out of the router and back in. Yep, login/password is obviously correct because Interent still works. So I go to the Internet Setup again, don't change anything, and just hit the "Test" button -- same error pops up.

Rebooted, same issue/err. "Test" button pops the error message whether I try to change a setting or not. In fact, not changing anything and just clicking save still pops the error.

I'm not sure what's going on because I had to enter these credentials when I set up the Orbi. I know they're right and they work or else I wouldn't have internet (they are the credentials necessary to connect the Orbi to my fiber modem).

Other things I've tried:

• I rebooted the Orbi.
• I tried changing the router language from "Auto" to "English"
• Tried a different browser

no changes with any of those. 

Things I haven't tried:

• Firmware update: It says there's one available, however I'm a little afraid to try it because I don't want to get locked out of my working internet if the firmware decides it doesn't like my login credentials. 

Anyone have any suggestions on what I can do to get this working? Thank you!