Gst clarification

My client is in to car services and has a branch in Bangalore and Hydrabad. He has got the registration in both the states.

He has tie-ups with few service providers in Hydrabad and they do provide the services on his behalf to his Hydrabad clients. He is issuing invoices form Bangalore to the Hydrabad clisents and collecting IGST.

Now the Problem is, as he has the Telengana registration, should and all the adminestration happens there, should he be charging IGST or CGST+SGST