Yep I got same problem; narrowed down, it is tied to phone at the same time.

I obtained 2 sim cards, activated them on different devices. My one device MiMax2 have no issues connecting, all smooth. The other phone a Redmi 4i (1 year old) have intermittent connecting issues. The connection shows H+, H and sometimes 3G. But no data connection gets through. Simultaneously as I switch other phone to tether, my this phone connects using wifi no issues. Ironically I tried this along my daily commute, faced the problem one day, but at other day the Redmi connected super fine.....

Seems it is network calibration they may be undertaking, cannot say it is phone exclusively...

got confirmation from chat support that Redmi 4i does not have LTE band 5 which is needed.