[2019] Co-op joining issues?

I'm having trouble playing Halo 5 with my mom, we just got a second Xbox and were excited to finally be able to co-op legendary the H5 campaign after doing all other Halo games legendary co-op, but every time we try to start, one or both of us gets dropped from the lobby, unable to rejoin. We tried going into the multiplayer, Warzone, Firefight, even custom games, and the same still happens. We're in the same house, is this a problem with us both being on the same internet? Problem with Xbox or Halo 5? Any help is appreciated.