Question [Q] No 4G, frequent signal loss

When i got the phone everything was working, when i flashed the uk unbranded firmware (instead of croatian t-mobile branded) I don't get any 4G signal and there is frequent signal loss when I don't have any signal.
Any fix?
I have been thinking of updating through pc companion, but I don't know if it will update to another uk unbranded or croatia t-mobile branded-which I want to try because of 4g and signal loss fix