Phone is intermittently switching to "Emergency Calls Only"

Hey there! Intermittent issues like that can be the worst. Never really knowing what to expect is one of my worst nightmares. It looks like you have already done a lot of troubleshooting to try to find out what is causing it to randomly drop internet. 

From what you already talked about, it makes sense that it is something on your line or phone. On the account side, there are only two features that would block out service for short periods of time like that. Family Allowances and FamilyMode are features that we offer that give time of day blocking. Double check your account and make sure you don't have either one of those on there. 

If it is not one of the Family features, then you are looking at something on your phone. I give you a huge kudos for already doing a couple factory resets and network settings changes to try and figure this out. A couple other things to look at:

• Third party battery savers or task managers - they can close down vital processes without consideration if they feel that too much power is being used. Double check the power saver options in settings. It is not as likely to close down vital stuff, but just to be sure the rules there are not too strict. 
• Updates - both software and apps being out of date can do a whole lot of chaos. From tiny things like slower response to touch all the way up to loss of signal and power cycling the phone. 
• Modo seguro - How to use safe mode | Configuración | Galaxy Z Flip5 | Asistencia de T-Mobile This will let you use your phone as close to factory as possible to further rule out any apps causing the problems.