Among all the other suggestions to fix it that didn't...
I upgraded from my S9+ to the S20 Ultra
I tried two more sim cards
I factory reset the phone WITHOUT restoring settings, apps or anything else.
I installed another update.
... and my husband's 3 year old S8 is getting 4G as I sit right next to him.
If another update isn't realeased soon to fix the issue, the phone's going back, and I'll have to break away from Samsung. I've been with them since my SIII.
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