Thank you for your response.
The first time this happened I was not able to get the messages with anything I tried. It stayed that way for several weeks. Then one day I got them. I do vaguely recall that I was at the place where it asks to load more, but I am not sure.
The last time, the messages appeared a day or two after I wrote this post, and I don't know know what I did to make that happen.
Yes, it loads messages from today (and succeeding days) once I get a connection. It also _has_ loaded the messages before the missing period, whether newly or from before I don't know. That is, when this has happened, only the messages from the time there was no connection were missing.
From what you said perhaps I didn't scroll far enough down to get the load-more option. I will try that next time. (It's not an obvious thing to do -- scroll down before messages you have already read to make it refresh ones in between.) The various refresh buttons (circling arrows) don't fix it. Maybe an option somewhere to reload (whatever the load-more option does), as opposed to refresh, would be more intuitive.
Thanks again for your good explanation of how it works.