I face the same issue. I use only 1 sim card and face the same issue of network fluctuations constantly. Also even if it shows 4G/LTE, the internet speed is considerably slower as compared to my previous device using the same sim card and number. It takes significantly longer to load the same page on mi A1 than my old phone. This is very irritating and disappointing from Xiaomi. Online transactions are endangered due to sudden drop in network. I request Xiaomi to  fix this at the earliest