Trying to put Orbi in AP mode, can't log into orbilogin

I just bought an Orbi RBR750 set from Costco and am trying to connect it to my AT&T BGW320 modem/router. After a lot of angst, I've realized I need to put the Orbi into AP mode. When I follow the instructions here:

https://kb.netgear.com/31218/How-do-I-configure-my-Orbi-router-to-act-as-an-access-point

I get stuck on point 4. No matter how many times I enter the correct password, I am prompted to reset the password. I do this, go through the two security questions, change the password, reenter the new p/w, and the same thing happens. I'm on an infinite loop on the p/w screen. I've tried on chrome and safari on my iphone and chrome and firefox on my macbook. Always the same thing. I'm connected to my orbi network, I use a lower case "a" for admin as the user name, I even tried "password" as the p/w and it always rejects whatever p/w I use.

Any ideas? It was frustrating enough trying to get this to place nicely with the AT&T modem; now the problem is absolutely on the Orbi side, since AP mode doesn't require any changes to the modem.