Two antenna issue

Hey! I have a TP-Link MR6400 router with SIM. If I use the regular bunny ears I get maybe 40 MBS down and about 10MBS up. I purchased an antenna to mount to the roof but it only had one cable and a splitter that went to two smaller connections so I couldn't use it on my router so I purchased a second antenna and mounted it on the same pole as the first. Now I have two cables and have wired them up but now I'm getting 3MBS down and 40MBS up! Any ideas?