Samsung S8 / S9 / S10 / S20 / S21 SMS / MMS not sending / delays receiving
1) Android Settings > Apps > Textra SMS > Battery > Allow Background activity = ON.
2) Android Settings > Apps > Textra SMS > Battery > Optimize Battery Usage = Exclude Textra SMS, so it reads "Not optimised".
3) Android Settings > Apps > Textra SMS > Mobile Data > Allow background data usage = ON.
4) Android Settings > Apps > Textra SMS > Mobile Data > Allow data usage while Data saver is on = ON.