I did not dig up the old posts on this, but I know a solution that works. Use a VPN, and it should solve your issue. There are things you can do with your modem (such as port forwarding) and whatnot, but this always fixed the issue for me on Project 1999 and on a few other private servers where I had issues. Here are some examples you can try -
https://www.bestvpn.com/blog/18736/5-best-free-vpn-2/ but I have a paid one. I know cyber ghost has worked for me in the past before I upgraded to a paid one. Let me know if it fixes it!