You are lucky. My Mi5 64gb is on Vodafone.nl (Netherlands) and all the available miui 8-version (clean install) don't operate on 4g. Now I am back on the latest miui 7 (stable), and the 4g-problem is gone. Yesterday I made a long trip by bicycle from the middle to the south of the Netherlands and passed a diversity of urban and rural areas; all the way good connections on 4g. So for me, no miui 8 anymore.