You need to update the motherboard BIOS to support the Ryzen 5 2400G processors.
The 3xx series motherboard can only run the newer processors after a BIOS update.
See this link for the BIOS version.
Also make sure that your RAM is on the QVL list for the motherboard.
More info on this... AMD Again Offers UEFI Update Boot Kits for Ryzen 2nd Gen CPUs