Permanent Solo Lobby Method

Improved, now with easier setup. No need to load into Change Appearance. Although, you're still fucked and probably have to use Change Appearance anyway.

Initial Setup
1. Open Windows Defender Firewall and click Advanced settings
2. Click Outbound Rules
3. Right-click Outbound Rules and click New Rule...
4. Select Port and hit Next
5. Select UDP, paste or type 6672, 61455, 61457, 61456, 61458 into the Specific remote ports section and hit Next
6. Select Block the connection and hit Next twice
7. Name the rule whatever you desire and hit Finish
8. Right-click the new saved rule and hit Disable Rule

In-Game Setup
Must be performed each time you want a solo lobby.
1. Launch the game normally while leaving Windows Defender Firewall with Advanced Security open
2. Do anything that puts you in a solo lobby, for example, enter a stranger mission, start a legendary bounty or load into change appearance
3. Once you're in a solo lobby, hit Enable Rule on the rule we just created
4. That's it, you're now in your solo lobby. You can complete your mission or exit change appearance and stay in. Enjoy!

Tested In-Game Setup Methods
- The dishonor-giving Stranger mission next to Lake Don Julio in New Austin (sometimes)
- Loading into Change Appearance, you may need to enter and exit PvP series to make it load (most consistent for me)

Adding Friends or Posse Members
1. Ask them to also set up the port-blocking rule on their own router or PC
2. While their port-blocking is disabled, send them an invite from your solo lobby
3. Have them accept the invite and wait for the to load into your session
4. Have them quickly turn on port-blocking so that other people cannot join your session
5. One of you may disconnect sometimes, just repeat the process to invite the person back into the lobby

Tips for Console Players
- Google how to block ports on your specific router or ask for help in this thread by listing your router
- The settings are likely under something like "Filter", "Security", "Firewall" > "Add custom rules" or something like that
- Use your router's admin panel to block the ports at the correct time as I've suggested in this thread (during Change Appearance, stranger missions, legendary bounties, etc.)
- If you can't figure out how to access your admin panel, just Google how. For mine, I just type 10.0.0.1 into my browser but it's different for other people with different routers.
- Guide for iptables: https://i.imgur.com/q6ASQsB.png

Troubleshooing Section
- Try restarting your PC!
- It doesn't work 100% of the time for me, rarely it loads me into tiny lobbies instead of an empty lobby. I just instantly go to Change Appearance because it works in small lobbies and try again.
- You may find yourself disconnecting a lot like normal if Rockstar's servers are under heavy load.
- It might help you to block the ports individually.
- If your lobby is auto-filling, you can type netstat -a -b in your Command Prompt to see which UDP ports are still listening and how to fix it.
- If you're disconnecting a lot instantly after getting into a solo lobby, you can try leaving your ports open until the lobby loads correctly and then block the ports after you feel like it's working. Just be wary that you might auto-fill sometimes and have to re-do it.
- For some people, just blocking port 6672 is enough and might work better for you.
- If you're playing with friends, there is sometimes an issue where camps and moonshiner shacks will not stay up. There is no known fix for this issue yet besides playing alone instead.
- Windows Firewall is garbage and sometimes requires you to restart/remake rules to get it working again.

If anyone can find a faster method, please tell us or post your own guide. This method was only discovered a few days ago and I'm sure it can be done even better if someone can find a way.