Welcome to the community!
Which version of the Mi A1 do you have? According to the specifications on the Mi Global website, that phone does not have any compatible LTE bands/frequencies.
As you note, the Mi 9 SE on the Mi Global website does appear to have two compatible LTE bands/frequencies. While the phone should work on the LTE networks, coverage would depend on corresponding bands/frequences on your surrounding cellular towers. You can get an idea of your area cellular towers here. In addition, the different bands/frequencies have varied characteristics. For example, the lower frequencies of bands 12 and 17 (700MHz) would be able to travel further and penetrate greater than the higher frequencies (see graphic here). Is it possible the Mi A1 has one of those bands and the Mi 9 SE does not?
Have you verified that the phone is using the proper APN? Since the phone is LTE-capable, it should be using ltemobile.apn. You can verify the settings here.
Hope this helps 😀