Hi, I have an MCPE server and have not found any mines yet. Maybe I was just unlucky, but in single player I found one directly. Could it be that there are no mines in Pocketmine?
PocketMine's generator is very basic. PocketMine's survival experience is basically nonexistent, considering there are no mobs either. If you want those things I recommend using the official dedicated bedrock server instead: https://www.minecraft.net/en-us/download/server/bedrock/