I'm going off memory here, so please forgive me if I'm not spot on.
There should be 2 connections on the bottom of the daughter board for your cellular signal. When your screen is put on, its pressing down on those connections. The first thing to check is those connections, making sure they are seated properly. If they are seated properly, then there is a chance that you tore one of your antennas when you pulled off the old screen. It happens, I've done it too many times. Check those antennas (you have to do some work to get them out) and see if you can find any tears. That is a very cheap fix.
There is a chance that the daughter board could be bad. There are plenty of used ones out there for dirt cheap. I've personally never seen this be the case, but it is possible. I would focus on the two connections and the antennas. Those are the simplest and easiest fixes and also the most likely.