Speak Telkom and get technician coming to your place. From your side do manual network search (again). If you see Telkom 4G on the list, then get a new prepaid SIM card (blue) R1. Prepaid SIM will connect to LTE if available, so you can check signal strenght. Post here, but not a number of bars, but real numbers from the router's information page.
If you don't see Telkom 4G on the list, then antenna will not help in most cases.