Since you are having problems in both joining and hosting, it may be an ISP problem. Civ 6 uses a peer to peer connection, this involves a high amount of pinging to maintain connectivity. Some ISP don't respond well to this. If you're using an ISP provided gateway, you'll have to forward the ports Steam uses. These are listed on a Steam support page. If you're running on a school network, you'll have to contact them to get the ports forwarded.