It is strange that the RCS (char service) on my s9+ stopped working recently. It was working fine when this feature initially made available through the July updates. By then I could send and receive chat messages through native Samsung message app to my friend who also had it enabled. But I noticed recently I noticed that all those message sent to same person who had the RCS enabled in their samsung message app have become normal SMS/MMS message. I have tried to toggle off and then toggle on and chat service switch, through appeared to be actived, the RCS has stopped working from the background. I am not sure if this is related to IMSI since my IMSI recently got re- provinsioned for Wifi Calling and Volte.
Try Google Messages app instead. It never failed me it always works well.
RCS has nothing to do with the IMS services no, so that's not the problem. The problem might also be with your friend, RCS need to be enabled on both parties devices for it to work properly. Maybe their service isn't always ON.