RBR20 won’t allow connection to Earthlink SMTP server

I have an RBR20 with two satellites. EVERYTHING works fine withe exception that I can’t send email through Earthlink’s SMTPAUTH.EARTHLINK.NET server. I’ve tested this using two different accounts on 2 iphones, 2 ipads, and 2 Win10 laptops. The iOS devices were tested using the native mail app and Outlook, and the Win10 machines used Outlook. If I turn off WiFi and use LTE, the Apple devices send OK. I swapped the Orbi router for a Linksys router and all is well. After talks with both Apple and Earthlink, plus three days of testing, it seems clear that the problem is somewhere in the Orbi RBR20. Can anyone shed any light on a fix?