Minecraft crossplay between two devices on the same wifi?

Depends which edition. Two Bedrock devices on the same wifi see each other automatically under the friends tab as a LAN game, as long as the host has multiplayer and LAN visibility turned on in the world settings. Java to Java works with open to LAN in the pause menu. Java and Bedrock cannot join each other no matter what wifi they're on, that's the one that catches people, so check both devices are actually the same edition before troubleshooting the network.