Do you live in a crowded network neighbhorhood - like in or near a large apartment complex? Have you checked your area for channel crowding? I would do this before spending money on more network devices. Use a sniffer to look at all the wifi signals in your area to see if the channel you are using is being used by other nearby wifi networks. I use
XIRRUS WiFi Inspector to see what wireless channels are in use and available.
NirSoft's WifiInfoView is another good one. Also popular is
inSSIDer.
If your networks (both 2.4GHz and 5GHz) are out by themselves, then I would leave the channel settings alone. But if other networks are using the same (or adjacent) channels as you, and the sniffer shows unused channels are available, I recommend changing your channel setting in your router, then see how your propagation goes. Note this setting is typically made easily in your router's wifi admin menu. You don't have to change anything in your connected devices, that will happen automatically.
Note if your full wifi spectrum is crowded, and all channels are being used, then you will need to look at the signal strength of the other wifi networks and move your wifi network to the channel where other networks are the weakest.
This is from almost 40 feet away on the other side of the house
40 feet "line of sight" should not pose a problem - even with 5GHz (though beyond that I would go with 2.4GHz). It is more about the number of barriers (walls, floors and ceilings), the composition of those barriers (thick concrete or rock, or thin wall board as examples), and the contents of those barriers (steel or wood studs, metal pipes and wires) that matters more. And then there are sources of interference such as other networks, other EMI/RFI sources like microwave ovens, TVs, cells towers, etc. Even reflective surfaces (like the metal side of a large refrigerator) can affect reception.
Another simple place to look is the antenna orientation of your WAP (wireless access point - typically part of a wireless router or residential gateway device). If your WAP has external antennas, you can try moving them around and using your sniffer to see if signal strength improves. Many antennas are detachable and can be raised and perhaps mounted high on the wall. If your antennas are internal, simply rotating the wireless router 90° may help. I would also try to move the WAP to a central (preferably upper) location in the house instead of on one end.
Adding a range extender or a second AP may be your only solution, but I recommend trying all of the above first before spending any money. Also note a range extender and/or second AP also adds complexity (and potential points of failure) to your network and the administration of it. If you have good house wiring,
Powerline Networking is another option.
I note a range extender or second AP are typically better for larger homes and/or homes with more than two levels. So I would look at buying a new, more capable wireless router before adding a range extender or second AP.
And FTR, these are all just good reasons to go Ethernet, when possible. Not to mention Ethernet is inherently more secure by default.