VOLTE gone after update, revert to Android P, EDL recovery done. Network stuck on EDGE

Please tell us some basic information before asking for help:
- Model Name: ZS630KL
- Firmware Version: 17.1810.2011.183
- Rooted or not: Rooted
- Frequency of Occurrence: Evergreen Now
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JIO volte worked flawlessly since purchase in December 2019. now no 4g/volte works, no jio no vodafone. same SIM cards working perfectly in other devices.
enhanced volte is ON, 'LTE only' in service menu too. but still stuck on EDGE/2g.
reboot, format, reflash android 9 and 10, nothing works
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Comments
This is happened bcoz you manually update your handset... Those builds for specific regions TW and US
I already explain this in my post to someone
WAIT for indian update.
Hi, we suggest you to please visit the nearest ASUS service center for better assistance. Locate here : https://www.asus.com/in/support/Service-Center/India
I updated to INDIAN firmware only. WW is global. I did not flash the TW version.
anyway, today i went back to my old home and i got volte back on Vodafone, so i changed the SIM immediately to JIO and volte is working now.
the problem is, that whenever i restart the device or turn airplane mode on/off, volte goes, and never comes back.
Some others as well are experiencing problems with 4G LTE. Some are stuck on 2G/3G while some have to switch to 2G to avoid frequent disconnection and reconnection.
Although I personally have not found any issues with the network, I'm using and Indian variant as well, but am experiencing other bugs, I'm thinking it will probably take a patch or two following this to perfect the functions.
Please stop suggesting everyone to visit nearest Asus centers, not all live near one. You suggested me the same but I had my issue solved with the help of some fellow ZF6 users. I'm glad I didn't just listen to you straight away.
Also, please try not to post this message so early on as others who might want to help will be discouraged after noticing your message.
Thanks,
Scooo.
UPDATE:
upgraded to A11, still no volte. Fastbooted back to A9 service firmware, still no volte. Neither on Jio nor on Vodafone.
WiFi calling worked flawlessly on A10 & A11 for a while, but gone after the reboot.
taking this shitbrick to the service centre tomorrow!