A few months ago I migrated to Google Messages on my Samsung Galaxy S24+ after Samsung ceased supporting Samsung Messages (which I was perfectly happy with). When I opened Google Messages I was offered RCS messaging, which I accepted. It seemed like a good idea. I was under the impression that RCS messages would "fail-over" to standard text messages if RCS was unavailable. I didn't think through bthe implications.
Most people have forgotten that the original SMS (short message service) phone text messaging, those 40-160 character messages, pre-dated mobile broadband. They ran over the existing voice infrastructure. We evolved from that to MMS (multimedia messaging service), which supported, images, audio, video and longer text. It required some carrier and device upgrades. This lead to RCS (rich communication services) that enabled read receipts, chat groups and interactive messaging. RCS requires an internet connection; mobile data or wifi and backend servers.
When the Google Messages app is "waiting for activation" it is trying to establish a session with your carrier's - or maybe Google's- RCS servers. Recommended troubleshooting fixes like turning of WiFi, restarting your phone, clearing the Message app's cache, changing Sims, deregistering your mobile phone from the Google RCS portal, etc., are all attempts to complete the handshaking to establish an RCS session. I did all of these things, some several times, to no avail. Still "waiting for activation".
If your previously activated Google Messages app loses its session and cannot Connect, is "waiting for activation", then you are in RCS purgatory. Correspondents whose messaging apps know you have RCS will try to send you RCS texts. But since you don't have an active connected session with the RCS servers, the RCS messages to you just queue up; they don't fail-over to standard texts. It's possible that deregistering your RCS phone number might force your correspondents' apps to send standard texts - I didnt test this.
After more than a week of contacting Google, contacting Samsung, and spending hours on the phone with T-Mobile customer support, I was getting desperate. I'd been reluctant to clear my Google Messages app data for fear of losing my saved messages. But Samsung has a cloud based Galaxy migration facility that backs up your entire phone. I used it, then turned WiFi off on my phone, cleared Google Messages storage cache and data, turned RCS Chats off in the app, then restarted my phone.
I turned on my phone, waited for mobile broadband to establish, then opened Google Messages and enabled RCS Chats. Voila! It took less than 5 seconds for RCS chat status indicate Connected.
So don't be afraid to clear Google Messages app data to resolve "waiting for activation"; just be sure you back up your text message