Orbi RBR20 is Blocking Email Messages From Being Sent

Hello everyone.  I'm new to the community, so please bear with me if I'm doing anything that does not follow normal protocol.  :-)

Lots of details to share, but hoping that someone might be able to help, as I've reached the end of my proverbial rope technically.

For 6 months, I had no issues at all with my orbi RBR20 system, primarily supporting (2) computers running MS Outlook, (2) iPhone and (2) iPads, running the Apple email client.  Then, about 5-6 weeks ago (yes, I've been dealing with this issue for that long), on a given weekend both computers and all Apple devices could suddently not longer send email through the Orbi router, neither wireless, or even one computer that is hardwired via ethernet into the Orbi router.  Receiving emails fine and no problem with internet connection... just sending emails.  Then, the troublshooting journey began.

I discovered that when turning off wi-fi on my cell phone that emails would magically send. I could also send emails from my phone when away from my house, or connected to another wi-fi network.  So, I tried deleting and adding accounts on all devices and computers (multiple times) with no luck, called Spectrum tech support four different times and after all that (along with some research online), have my crosshairs set on the Orbi.

This was confirmed after changing out my Spectrum-assigned cable modem for a new one (model# Hitron EN2251), and even removing the hardwire from the Orbi router and plugging it directly into the cable modem, emails sent as they normally should.  Then I hardwired my computer back into the Orbi and the sending of emials came to a grinding halt.

So, I tried power cycling the Orbi system, and amazingly, a few test emails from my computer slipped through my outbox, but shortly afterward, no more emails were able to be sent.  Tried another power cycle, but no luck at all getting emails to send.  Finally, did a factory reset and started from scratch, installing the Orbi system as if it had come straingth out of the box.  No luck at all with emails being sent.

Next, was my call to Netgear, wondering if there might be a hardware failure that would be covered by the 1 year warranty.  After answering some questions by the rep, it was determined by them that the Orbi was fine and that there must be some router settings that would need to be made in order to rectify the problem.  It was then that I was informed that my complimentary 3 months of phone support had expired and that I'd be looking at paying $89 for a 6 month "support contract" to cover all of my network equipment.  Are you serious right now?!  I don't need a 6 month contract.  I don't need coverage on all of my network gear.  What I need is a 10-15 minute conversation with a tech that can tell me to adjust some router settings and tell me to go on my merry way!

Again, it is glaringly obvious that the ultimate solution to this problem is in some router settings, but unfortunately, the reseach that I've been able to do, combined with my mediocre technical level have led me to asking for a life preserver to be tossed my way.  Any help that might be able to be provided would be GREATLY appreciated!