i have the same problem with my Huawei P9 Plus, especially after I swapped to my 2nd sim to take on the 4G data services. After that, I swapped back to my 1st sim which is the webe sim. It interminttently can't latch on to the webe 4G network at all. So I had to manually select the webe 4G network to latch on.
I then turn on the automatically select network mode, then it works fine.