The Mosqitter Grand connects to 2.4 GHz Wi-Fi only — it doesn't support 5 GHz networks. If your router broadcasts both frequencies under the same name, you may need to temporarily connect your phone to the 2.4 GHz band during setup.
Tip: most routers let you separate the two bands in the settings (for example, 'Home' for 2.4 GHz and 'Home_5G' for 5 GHz). Just connect your phone to the 2.4 GHz one before pairing.