Default phone issue, wifi, network

hi guys,

im looking for a little help if you dont mind. my z ultra is having issues when i dont have a wifi connection. while i have a wifi connection all is good but when i lose the wifi i am unable to access the net and browse unless i manually go into my setting and turn wifi of so the phones network ( three ) takes over and again then all is good.

do i have a setting wrong on the phone as surely when the phone loses a wifi connection the network ( in my case three ) should automatically take over and i should not have to manually turn the wifi off for the network to kick in.

any help would be great please.