fyi.
Unfortunately, ASUS doesn't expect customers to upgrade these complete desktop systems, or doesn't provide much support or information about it as you may have found.
If you check the ASUS G11CD download site though in addition to the 0905 bios for the normal G11CD, there is also a 0602 bios for the 'Kabylake' cpu which your i5-7400 falls under, but it's really for a different G11CD motherboard, so unfortunately, it sounds like your system may not support the i5-7400 if you want to check into it more.
If you try to flash the G11CD-K 0602 bios, you should see the system will not let you since you don't really have the correct motherboard or system for that bios (and cpu).
You may want to check around more to confirm this and maybe check that cpu to confirm it is good if there is an easy way (pretty sure it is ok), but that's my understanding of the situation although I have not tried it myself or checked much more into it.
Sorry for the trouble and hope that makes sense. Hopefully it can save you some time and trouble at least. Thanks.