Hardcore works but the second I hit core, it either kicks me to the main lobby saying it wasn’t joinable, or puts me in a game and then the load out clock stops somewhere in the 20s then boots me to the main lobby. HOW DO I FIX THIS??
I've ran into this issue quite a bit myself recently, and have only found a couple of temporary "workarounds". (BTW: Are you selecting modes listed in the "featured" playlist, or are you selecting from the core/hardcore tab option?)
When I play the featured playlist. I can usually get into whatever mode I choose first. If I leave that mode for any other game type, the lobbies do not populate at all, and I get the unjoinable lobbies error, I have to close the app, and relaunch. This usually results in me being able to at least change game modes, but it's a pain in the butt.
The other option is to restart your entire network;; console included. Make sure that once you shut down your console, you unplug it for 30 seconds or So, in order to clear the cache. Sometimes this works, usually it doesnt. Either way, it's a good thing to do.
For what it's worth, I've found that if my in game bandwidth is below 4000kb, the game plays like garbage. Sometimes, simply restarting the app solves this issue, while other times I have to reset everything.