If the game doesn't appear in the LAN tab, disable Windows firewall or add an exception for Anno1800.exe on private networks. Also ensure all players are on the same game version (including DLC matching—everyone must have identical DLC enabled).
: You can set up a "4v4v4v4" match with 16 total humans, or play as a co-op team of four against three separate AI opponents. Setting Up a Session Anno 1800 (PC) Co-Op Information - Co-Optimus anno 1800 lan multiplayer