The best alternatives to Yare.io are Squarecraft.io, OpenRA, and Battle of Flags. If these 3 options don't work for you, we've listed a few more alternatives below.
Squarecraft.io is a real-time browser-based multiplayer strategy game. Players direct armies to capture and defend towers, all while racing to collect 20 flags. The game offers both a global Free-For-All queue to battle and earn ELO ratings, and a lobby for casual unranked games between friends. Discord: https://discordapp.com/invite/ZjDHYch