It may not be possible. You can look the internal photos up at the FCC database since it appears this one is sold in the USA so they registered it. What you will see is there are what appears to be 3 antenna connections with one marked as wlan.
So all you can do is try to connect to the other ones and see what happens. The connectors are standard but I do not think they are ts9.
Now it is likely illegal to do this. It depends on the country also I suppose. In the USA you are allowed to add antenna and amplifiers etc BUT with the restriction you must get permission from the vendor. I assume this means you must have a legal document. The thing is no cell provider is going to want to deal with this since now any interference is also on them as well as you. So pretty much even though there is a exception you will never get it so all these devices are likely illegal. They just do not enforce the laws until your equipment causes a issue....and then they stomp you the penalty is huge.
I am going to bet since they do not make it easy on this device you have no chance to get this done legally.