Seeking recommendations for unlocked VoLTE phone that AT&T will not support after 3G phase-out

As for your thoughts

• Fight to enable VoLTE in the network (it's enabled on the phone, but not in the network)

There is no point oh, if the phone is dropping to an H or an H+ currently it is not voice over LTE capable on AT&T.  That indicates it is dropping to HSPA which is 3 G. 

• Fight for a comparable replacement phone (an S10, 20, or something close)

AT&T will be perfectly happy to send you a free crappy Android. Otherwise you are on your own to either upgrade, which requires postpaid service of course. Or purchase your own phone and bring it to the network as long as it is on the whitelist

• Switch to T-Mobile. Supported but I worry about the same thing happening later.

It's very possible the phone will work on T-Mobile. They are set to shut down around the same time as a t and t. So either the phone will work, or it won't and you will be in the same fix. 

• Switch to a different alternate carrier (Verizon not supported by this phone, but there's probably others I could flee to)

There are three major networks wish you have mentioned. Almost everything else is a smaller mvno of those three major networks. When 3-g shuts down, it's going down for all of those mvnos as well. 

• Buy a new phone and sell the old one... (Edited per community guidelines) it up as an expensive lesson learned in how carriers support unlocked phones

It's already a two generations older phone. While I'm sure it's working great for you, it is a two generations old phone. Your best bet is to get whatever cash you can out of it now by selling it out of the United States where it is likely to find a home with someone who can use it. Especially those dual Sims are very popular in Europe and Asia. 

• Scrap it all and start over fresh with a new carrier and device

I don't know why you would scrap it it's worth something on the used Market.  

Whether or not you switch carriers and use this device or get a different one is entirely up to you

You would be wasting your time with options 1 and 2. You can’t fight to enable VoLTE, only Samsung can do that by submitting the phone for certification testing which they probablywill not do. As for option 2 ATT isn’t going to provide anything but a cheap phone. Not sure why you call it the 3G/2G phaseout, 2G was shut down years ago.

@sandblaster : thank you. I thought that there were still parts of 2G available and being shutdown too...

Anyway, thank you re: my first 2 options which is where I was going to start. I thought that AT&T was simply being obstinate about not enabling VoLTE on my phone (they also fumbled through enabling 4G data for me as well, but I got it going eventually).

My inclination at this point is to switch carriers so I don't have to buy an expensive phone, switch over, and sell my still quite capable phone.

If you're looking for an unlocked phone that will work on AT&T you need to work with the whitelist which is available here.  There may be a couple of errors or typos, but this is pretty accurate.

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.att.com/idpassets/images/support/wireless/Service-Capabilities-Unlocked-Devices-ATT-Network.pdf&ved=2ahUKEwiImeqyqtPwAhW9KFkFHZXWAoIQFjABegQIGxAC&usg=AOvVaw1C5znQishzVLbCB1H7kNIA

@lizdance40 : thank you. You are confirming much of what I suspected.

I had the white-list already, although I'm not sure it existed when I acquired this device. I did my homework on compatibility at that time but got bit anyway.

FYI, it runs at 4G most of the time, but does drop to H/H+ occasionally. Not sure how voice is handled... but I have verified that despite VoLTE enabled and capable, it doesn't work on AT&T. It never indicates "LTE" as a status. This has been generally workable for me albeit troublesome at times. Maybe T-Mobile can guarantee to enable it in which case it gives me an option.

Your best bet is to get whatever cash you can out of it now by selling it out of the United States where it is likely to find a home with someone who can use it. 

I think this would indeed be the best avenue to pursue. Thank you for the suggestion. The main bummer is the hassle and cost (the S10e was $650-800 when I bought it and I end up going through it again about 1.5 years later.

I don't know why you would scrap it

I meant AT&T and the whole situation. Start over fresh like I'm a teenager with a new phone, carrier, and plan. :)

I'd still sell the S10e as used.

Thanks for spending the time on this. I appreciate it.

Very welcome.  Good luck