Leon LTE (MS345) Problem: "Searching..." forever but SIM is OK

Just posting this in case somebody has some ideas. I've had a MetroPCS LG Leon LTE (MS345) for over a year without problems, but now it started losing its mobile connection sporadically, for a few minutes or a few hours, as if there was no SIM in the phone.
Symptoms:

• The 4G indicator disappears from the notification bar (signal strength indicator shows good signal).
• The lock screen says "Searchng..."
• Calls fail with the "Emergency Calls Only" message
• SMS won't send
• I get a "Service Unavailable" error when I choose Settings/Call/Additional Settings
• WiFi is unaffected (and WiFi calling seems to work, though it has always been hit or miss)

Things I've already tried:

• Checked relevant settings, APNs, etc
• Checked the contacts on the SIM socket, all looks good.
• Reset Phone to factory defaults
• Replaced SIM (MetroPCS did this for free)

New phones are relatively cheap, but I don't have a burning desire to upgrade, so I'm hoping for another option.

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