i recently updated my server version to 1.19. But now MCPE 1.18 version players unable to join my server. i have added Viaversion / ViaBackwards and Viarewind plugins in my server. But still 1.18 version players couldnt be able to join in my server.

This issue should go to geyser's issues list but will answer it anyways;

Geyser no longer supports 1.18.x bedrock and instead must be updated to join - If your server is still older than 1.19 then you can use viaversion to allow geyser to properly emulate 1.19 java clients.