BajanK You can try leaving the EA App in offline mode and keeping your computer online. If that works, you can put the EA App in online mode but disable Online Social Features within Sims 4, under Options > Game Options > Other. This would cut off your access to the Gallery and time-limited events but not otherwise affect the game.
As for what's causing this, it could be your antivirus or something about your connection. If you use a third-party antivirus, try playing (online, with all features enabled) with the AV disabled. As long as you don't do anything else at the same time, your computer should be safe.
To test the connection, try using a different one. You could take your laptop to work/school, or a public space with free wifi, and see how it goes. Or you could set your phone as a mobile hotspot, using data rather than your home wifi, and connect your computer to that.