Are you sure? Do you have this phone? I know the specification, but some reviews claims that the phone has no LTE band 38. Example: http://www.gadget.ro/umi-emax-mini-review/ , see first comment.
On official site, no word about LTE band 38.
I also own this phone, and we got here B3/B7 which is in specification, but it does't work. Only 3G+
Whatever I try, no 4G, which is sad.
Umi iron and umi super works in 4g here without problems.