Text messages sent to a phone offline are not delivered

I have added some additional details below.

There appears to be a bug in Xfinity Mobile's SMS delivery.  I have tested it to see if it is carrier-specific, however it is not.  I specifically disabled RCS for this test.
SMS have been sent from my 3a (online) to my wife's (powered off), and if more than an hour or so elapses, they aren't delivered upon powering her phone back on.  I have run the same test with SMS's from an iphone on the AT&T network, my son's iphone (Xfinity), as well as from Google Voice. The sender does NOT receive any error messages, like "text undeliverable."

This can't be solved by simply leaving the phone on all the time.   On a flight that is multiple hours long, texts that are sent to me during that flight will NOT be delivered to my phone (if they are sent >1 hour before I land).  I will never know that they were sent, and the sender will never know that they weren't delivered.

My understanding is it is standard procedure for the carrier (Xfinity or Verizon or whoever) to store undelivered SMS for at minimum several days, re-attempting to deliver at a given time interval until they are, and if not, discarded after several days, perhaps up to a week.  In the case of Xfinity, it seems this SMS "storage time" is less than an hour, which is not expected, and probably should not be the case.

Please clarify if this is a bug or not, and if so, please help fix.

When you enable or use RCS, Textra, or other advanced messaging on Android phones your non SMS and MMS messages are probably not being stored on XM/Verizon servers. I understand that that there have been many issues with RCS implementation because each carrier and each phone provider are still doing things somewhat their own way. One of the problems has been that once one of these advanced messaging applications have been enabled on some phones, some users have not been able to entirely disable the advanced messaging (return to original SMS and MMS phone software) without restoring their phones as new. Ugly. Unfortunately there is no way to determine whether Textra or RCS has caused the problem, or whether it is actually XM/Verizon SMS and MMS servers. 

Here is an interesting article on RCS: https://9to5google.com/2020/06/19/google-messages-disable-rcs-form/

It appears to be SMS related.  I had a friend with a Samsung Galaxy on T-Mobile send me a message last night when my phone was off and it arrived this morning when I turned the phone on.  He had RCS enabled as do I.  Both my sons have iPhones and when they send SMS messages if my phone is off, they never arrive.

I looked through all the settings on Messages (Google's Android texting app) and I see no setting that would block SMS messages.  What I would like to know is how long is an SMS message that can't be delivered immediately kept on the XM/Verizon server before it is deleted?  From my experience, this is less than 8 hours.  I know that for AT&T, it is 3 days and delivery attempts are made periodically during that time.  Prior to subscribing to Xfinity Mobile, we were on Consumer Cellular which uses the AT&T network.  We never had a problem with SMS delivery, even when the receiving phone was off.  The phones were the same ones we are using now.

My mom’s phone (on my plan) can’t dial out or send or receive texts. Her number is [Edited: "Personal Information"]. Please help.

I also cannot receive texts if they were sent when the phone (Google Pixel 4a 5G) is off. When the phone is turned on, no messages appear. This happens even if the phone is turned on immediately after a message has been sent via SMS from another phone. I never receive the message. I can receive texts when the phone is on. I only have the default Google messaging app on the phone. How do I fix this? It is unacceptable to have to have the phone on at all times.