My friend can host but can't connect

[Kirbington]

Tried playing Mario Party 7 with 3 of my friends last night, and while friend A and friend B could connect to my session fine, friend C never managed to connect to my session, or A/Bs. We were all able to join friend C's session without issue. We noted that friend C has a different ISP, which might be the root of the problem.

[Kirbington]

I was able to fix this by turning on the dmz on my router settings. In case that doesn't work, messing with your router and/or windows firewall settings might help too