HT503 Refuses to Register

I have two Grandstream HT-503s. I’m following the wiki post:

Configuring HT503 as FXO

Following the exact same process, that is, setting up an extension then setting up the HT-503 to connect using that extension and password, I get one of my HT-503s to show as registered under the HT-503 status page but not the other.

I am using two different extensions, one for each of the HT-503s. I am making sure the SIP user id and Authenticate ID match. I have reentered the password a dozen times. But still, one shows registered on the status page and the other does not.

Any ideas?