I did some researches in this forum and I found some information below:
Yes, I gave them myEID number in settings and AT&T imei and SIM card and they reprovisoned it sent me back to cellular activation and then 5G became an option. Took 20 mins with their chat. Just message them and say I need my e sim re provisioned to work with 5g
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Back to iOS 14.5, there is the same issue that 5G option is not shown up. More than two people said, they contacted Att and updated myEID number (MEID?), then their phones worked. I only updated IMEI and ICCID recently, not MEID. Maybe that is the reason why it didn't work. I will contact Att to update MEID and I will keep posted.
Update: Finally I got it worked. Because online chat was off now so I called att directly. The agent said they don't use MEID. The agent said EID number is for eSim and IMEI is for physical sim. He said EID number is a very long number, and my provided MEID didn't work. I don't agree with him on this point. Anyway, this agent asked me for the ICCID and IMEI. He asked me to power off my phone. then he updated my sim card and enabled 5G again (third time try in recent two days). Then he told me to power on the phone again. This time, 5G option shown up.
I really don't know what is going on. Maybe support line is better than online chat?
In a word, if you have the same issue, you need to contact att to "reprovisioned" your esim card. I tried to call att and it worked. I tried to use online chat twice and neither worked.