You could just stop playing with the screen brightness and volume control while you are working... and leave the WiFi on all the time.
What work critical functions do you need that were controlled by a Fn combination? Most importantly, if your Fn key is critical to your work, why would you NOT take the time to get the laptop repaired properly?
Chances are, what is wrong with your laptop is an issue with the keyboard itself, as opposed to an issue on the motherboard. What this means is that this is how your repair job would go.
Your laptop keyboard is removed to be examined, as well as to allow access to what is beneath it. This is to look for any evidence of a spill, or a loose screw, or burn/scorch marks, or a damaged keyboard cable connection.
Barring any visible damage to the motherboard or evidence of a spill, your laptop keyboard is replaced with a new one. If everything works, you are contacted, and you pay. Depending on the shop, they may replace your keyboard with a temporary "tester"... a keyboard made for your laptop or compatible with your laptop, but just not new and one they keep on hand for just such an occasion. Having one means they can potentially avoid purchasing a new one if it turns out that your issues are not this simple.
If a replacement keyboard does not do the job and fix the issue, then the problem is FAR more serious, and might involve requiring a replacement motherboard, and/or replacing the entire unit. Yes, I know. Seems silly to replace a whole laptop just because the Fn key stops working. Yet, if the keyboard itself is fine, and the problem requires the replacement of the motherboard... the cost of such a job could exceed the cost of a replacement laptop.
Very rarely do laptop keys just stop functioning without there being a reason. Almost all of the time, there was a liquid spill of some kind, and the user refuses to acknowledge it because the spill was very small, or the user just flatly refuse to believe that a little Iced Tea could possibly cause the key in question to stop working. What they don't realize is that Laptop Keyboards are not made to be repaired. They are made to be replaced. They also don't realize how keyboards in general function and/or are constructed. They don't realize that more than half of the laptop keyboards out there have a grill-like construction... so that anything spilled on them will pass right through them and onto the more sensitive components below. Here are two laptop keyboards made for the SAME model laptop...
... one with a solid back, the other with the grill-type. Why is this important and being mentioned at all? Because whatever caused the Fn key to stop working could also cause the whole laptop to stop working. Not getting it repaired as soon as possible could make the difference between replacing a keyboard, and replacing the laptop.
What's cheaper. Losing a day or two of productivity, or replacing the laptop?