I already have inssider 4 lol
My range use to be a good bit better with the same firmware and the same router. I think there's something causing interference but I have no idea what it is... I've tried different things... nothing seems to make a difference though.
I just don't understand it
I am not sure I understand your problem, here are some thoughts:
Sudden changes in wireless signal quality are often caused by external factors like new neighbor equipment or neighbor equipment using another wifi channel. Here InSSIDer is your helpful friend.
Further the climate plays a role (humidity for behaviour of the radio signals and temperature for behaviour of the router hardware) and obstacles between the router and the wireless client (did you move the router, or did you move obstacles?). This is something you have to determine yourself.
A bad powersupply may play a role by e.g. generating noise or supplying a low voltage, a little more difficult to determine.
Finally the router hardware may be aging.
Firmware with different wireless drivers will for quite sure have an effect on the wireless signal, either the signal strength or the throughput or what ever.
The wifi radio signals (multiple, because your router sends out multiple signals at the same time) are quite complex signals with many variables.
In addition, the firmware has drivers for two different radios: one for 2.4 GHz and one for 5 GHz. When both drivers are updated, you may find different effects on both radios.
For real good testing of just wireless drivers, one shall make firmware builds with just one driver changed at a time, and no other changes (as far as feasible). In the real world developpers allways make more changes in released firmware builds, and for wireless drivers they are bound to what the chip maker (Broadcom in this case) issues. The wireless drivers are part of a Software Development Kit (SDK) issued by Broadcom (not even Asus), that contains more then just the drivers.
Last but not least: the RT-N66U exist in different hardware versions. Have a close look at the product sticker for differences in models numbers etc. Maybe you shall post photos of both here (wipe out the MAC address).
I still doubt if different RT-N66U router hardware behaves the same with the same firmware builds.