First thing you should do is return the 2400G and get an 1800X, you have dual 1080TIs, a 2400G isn't going to cut it. That will also solve your boot problems.
- I think only Gigabyte AB350N comes with out of the box BIOS support for Ryzen 5 2400G (not 100% sure). I tried entering your specs in PCPartPicker & it says there might be compatibility issues with Ryzen 2000G series processors unless BIOS is updated.
- Just to ensure the power supply is not shutting off due to over load (very unlikely as I see the wattage is less for the new components), could you remove all the graphics card from your motherboard and try powering on.
- If nothing works I guess you need to flash your BIOS. For this you will need a new processor as your motherboard supports only Q-Flash. Most likely AMD will ship a different processor to flash your BIOS but you will have to return it . Please find the link Unable to Boot New Desktop System Configured with AMD 2nd Generation Ryzen™ Desktop Processor, and AMD Socket AM4 Mother…
- If you cannot wait that long, then you could return your current motherboard & get another one with Q-Flash Plus - this enables BIOS to be updated from a USB without the need for a processor or RAM. This is again if you find Q-Flash Plus enabled motherboard for AM4.
Please note, I am in the processing of researching on the parts needed for my Ryzen Build. I have not built one yet. Please keep this thread updated if you find a solution. I am curious.