Take it outside and see if you get any signal. Even when you connect a external antenna it is actually using both antenna at the same time it just prefers the external antenna because it gets more signal, the internal one was never even disconnected.
It really depends on the band your carrier is using in your area. The high frequency bands have lots more issues getting signal into the house. Most times people put those mobile hotspots in the window to get the best signal when you do not have a external antenna.