Thank you for the two links!
I managed to get 4G working by fiddling with the settings in the Ubuntu Touch UI, but I didn't really understand what I had done.
However, your two links led me to a lot of helpful pages that made me better understand the Ubuntu Touch APN interface and which led me to discover where the APN settings are saved.
The current APN settings are found in a text file: /var/lib/ofono/<your_imsi_number>/gprs, as mentioned in this message.
Unfortunately, one cannot directly edit these files merely by being root with "sudo su", as they are inside the "read-only" file system. On the other hand, the linked message above also details a quick method of manually editing the APN settings within the file. I tried that method by making an innocuous change (inserted a space) in the gprs file, but when I restarted ofono, I found that I lost 4G for awhile. Perhaps it was because the service was merely overburdened at that moment.
You can also edit these and most other files read-only files by changing the mounted read-only file system to "read-write." At the adb phablet-shell prompt, use "sudo mount -o remount,rw /". Make your changes to the file(s), and then do "sudo mount -o remount,ro /" to change the file system back to read-only. You might have to restart ofono ("sudo restart ofono"?) after you make any changes to its files.
I tried this remounting method to edit a normally read-only file (but not an ofono file) with an innocuous change, and the phone didn't blow-up. Other than merely having root read-write access, if there are any inherent dangers in converting the mounted filesystem to read-write and then back to read-only, somebody please warn me now.
By the way, if you had other sim cards inserted in your phone, you might find multiple "IMSI" directories in /var/lib/ofono. There are several ways to find out the IMSI number of your current sim card. One easy way do so is to run /usr/share/ofono/scripts/list-modems and look for "SubscriberIdentity = <your_current_IMSI_number>" near the end of the output.
You links also led me to what is perhaps the "proper" way to manually set a custom APN, using the ofono scripts.
If anyone is interested in the designer's thinking (overthinking) behind the Ubuntu Touch APN interface, I followed your links to this page and this page with development sketches and discussion on the layout. I briefly scanned these two pages which helped me understand the interface a little better, but I think I will eventually use the manual read-write mount method to clean up the APN settings, if there are no dangers involved in doing so.