What are you using for SIM card provider?  I tried some real low cost cards off amazon (Spectrum Sim and Kiwi Sim) and they worked sporadically.  They would connect and then drop the connection and then sometimes reconnect and/or bounce off and on.  My base software would say it was connected 100% but I think it was getting data packets messed up.  I was told it is because they route their data through foreign servers.  Got a card through Simbase and it has worked perfectly.