Possible solution, but it's scattergun... and too numerous to detail.
Firstly, going thru some other forums, for HTC... someone reported they completely disabled Doze(app by app - the only way when not rooted) and didn't lose too much time between charges: from 6 hours down to 5.
Yesterday i went through all system(mainly) apps that could have anything to do with Data. Wound up with 60 Doze-disabled - 59 others in addition to the modem thingie.
Alas, still lost connection more than a handful times- including once during a Google Maps trek, another stubbornly upon reboot.
So I went back and disabled a lot more system processes - including things that deal with alarm, notifications, location, Bluetooth, processor, ram, lockscreen, fingerprint, calls, messaging, volume, sync, etc... plus Google stuff-services, framework, etc... and stock apps that communicate with web and / or have acted buggy and/or help wake up phone to connect to web...
I didn't touch processes that can't really help - wifi for example.
I must have a couple hundred off Doze.
Passing the 12 hour mark everything OK, but too early to tell for sure.
Don't worry, you won't break anything. You lose maybe 15% battery life...and at worst you could suffer a significant memory leak with any app not properly recoded for Doze - look with suspicion at anything not updated in over a year.
You can always reset ALL app defaults with a single tap.
In my opinion Google pulled a Microsoft - Doze didn't improve battery life all that much, made it almost impossible to disable & hid most its settings, plus a lot of independent apps(including carrier) have problems with it.
I'll keep updating results, and if i get another disconnect - will look to see what else can be disabled.
But it's looking encouraging. For example, upon waking up, the latest emails are waiting in Notifications - instead of the phone being without a signal.