If the host didn't port forward then do it as it's the only reliable way to make it work all the time.
If he did then most likely his PC's local IPv4 changed which means the rule he created before isn't assigned to his PC anymore so he needs to update the rule (and set a static IP if he doesn't want this to happen in the future)
https://plutonium.pw/docs/custom-games/#if-upnp-is-still-disabled-or-you-cant-connect-despite-it-being-enabled
undefined
