Originally posted by xun1013:

為什麼防火牆我與好友全關了還是不能與好友連線遊玩 版本設定也都是一樣 卻還是出現防火牆問題?

The window noting a fire-wall error after the fire-wall is turned off, can be ignored.

If for some reason the Multiplayer is still not functioning; here's a discussion where most of the info on work-arounds with Lan / Port Forwards have been noted:
https://steamhost.cn/steamcommunity_com/groups/TLM2H/discussions/10/2785991200453324508/