DirectPlay reminds me the work I did 15 years ago on it with dplay7info, dplay8info, dplay7fp, dplay8fp, the dp8games information and some security vulnerabilities.
Personally I can't help and it's impossible for me to remember that old stuff but I want to give an answer just in case nobody else will.
The difference you see between Outlaws (works) and Redline (doesn't) may be just related to the DirectPlay version used in these games, 6/7 and 8/9 are very different.
Are these 2 computers on the same LAN or on the Internet?
(you mentioned a router so it's definitely Internet)
Are you using a VPN software? And what?
(I remember that LAN was mandatory and was necessary a VPN for playing games, like Hamachi)
Have you tried to collect packets with Wireshark for checking what gets lost?
In my opinion, any answer is better than no answer lol. And you actually helped me with 1 thing: the fact that 6/7 and 8/9 are vastly different. I didn't know any better.
What I've been trying to do, is using my desktop PC via my home router (Win10) and using my laptop via my phone's hot spot (Win7), as I wanted the connections to be separate. In addition, I had a friend in another country try to connect to me either thru my laptop or desktop PC (just in case the phone hotspot was the issue during my experiments), and neither one of us could connect to each other anyway.
The only VPN I attempted was Kali, if that counts. I was getting the same results after a few hours, and so I gave up there. I knew VPNs were needed for emulating LAN games, but didn't think for TCP/IP too (although probably possible)? I also tried hooking up both the laptop and PC to the same network, and did an IPX session (had to use ipxwrapper though because ipx is dead n gone in Win7+). That works perfectly fine. And so at least I know it's not a game version issue. I also tried both Kingpin (1999) and Rainbow Six (1998), and those both connected just fine. I'm only bringing up those 2 to show that I tried games that don't utilize DirectPlay, though.
I have never used Wireshark before. I'm looking at it now, and maybe that would give me some insight, if I knew what to look for. Thanks for the suggestions!