iPhone signal disappear always to be no service in the buildings!

hello dears

unfortunately my iphone 5s has his signal disappear especially in buildings

and i've changed my sim card and tried my sim card on another device

and it works fine

so the problem in my device but it seems its not a hardwere problem because

when i open airmode and turned it off again the signal come back

then goes again so if anyone has a solution to my problem

thanks for your cooperation