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I'm not sure that this is your issue though. From what I remember you only need a GSLT if you want your server to act like an official one. For example I still have an old private MvM server running without a GSLT and can still connect fine. And this server is showing gmsindex as -1 also. However this server is connected to the internet with a public IP and not behind a private network like yours.
I would check if using a GSLT makes it work. If not it's most likely a port forward or firewall issue.