At&T Galaxy s5 Wi-Fi hotspot

are you talking the att app?  if so ya it detects the att sim

I am talking about Teathering and wifi Hotspot in settings I am using t-mobile. when i try to enable it it says "please wait while verifying" and it is enable from t-mobile side but it seem to me like Phone is try to verify with at&T server 

WiFi hotspot and tethereing  will not work on other networks with different phones (HTC One X and One M8 are my specific experience), so I suspect this is the case with the S5 as well.  With root it can be made to work on the HTC phones.  So if you need it, you should research if that is true for the S5.

The hotspot section is also a little tricky, unlocked phones need to have the T-Mobile settings programmed in and the hotspot settings may not be editable to include network's information. In this situation, you need you to get in touch with Samsung / AT&T / T-mobile to see if there is a way to get into that part of the network settings and change the programmed info.