OK, apparently you realise that the problem may be connected with lack of device drivers. While the manufacturer hasn't released drivers for Windows 8.1, at least Intel has them. They may be more generic than from the manufacturer, but at least you can try: Downloads for Intel® Z390 Chipset
Another possible way: the manufacturer might have released Windows 8.1 driver for another board with the same or very similar chipset. With some luck, it may help too.
As for network controller, try to find its manufacturer and download a driver there. Alternatively, you can look at the hardware ID and search in the Internet your ID or any compatible (Device Manager -> device with driver problem [!] -> Details -> Hardware IDs and Compatible IDs).
Of course, an installation of non-specific driver poses some risk, in worst case a BSOD during the boot. Don't forget to create a system checkpoint before installation or, even better, a full system backup on an external drive.
Sorry, I haven't understood the situation with updates: have you already installed all security updates? Look at Windows Update, View update history: it should show more than 100 updates in Important category and some other in recommended. If so, the list of current important updates should be empty, only some optional updates may present (if you decided that you don't need them). I ask because it's quite common that after a fresh installation of Windows 8.1 the automatic update system doesn't works properly, so need to install a specific update manually which resolves this problem.