Back when I got started we had to be concerned with s registers and with Hayes AT commands. The software (and firmware) interfaces which exist today had not yet been developed.
Today, I don't know 99% of what is going on in the transfer process.
But one rule still works: "Have you tried unplugging it and plugging it back in?"
My mobile hotspot often needs me to disable and then reenable LTE. Tends to fix the problem most of the time.