Do you want to just extended the WiFi signal, or install another AP?
These are 2 different things.

I’d suggest installing a second AP is a better solution if you can run an Ethernet cable from your current WiFi router to the new AP. You can have APs from different vendors, but they would not be able to cooperate with each other to handoff users (one of the features of the Ubiquiti APs).
Edit: I should add that users can switch between APs just fine if you configure them with the same SSID and passwords. However, that is something that the client does and takes a bit longer than if the AP detects your signal is weak and then informs it to switch to another AP. However, I have not used that feature myself since I only have one UniFi AP.

Range extenders are not efficient as it reduces your throughput by 50% since all the packets need to be sent through 2 devices. In addition, I don’t think it is a good idea to mix equipment brands for this kind of usage.

Tbh it wont improve your range much if you have a bigger house. Its better to run both asus and the uap at the same time. You just need to wire the uap.

Little tips for AC LR, set ur region to Australia to get the maximum 5GHz tx power, not US. I used US for both of my nanoHD and AC LR, and wonder why 5GHz in the LR is few dBm lower than what I set but nanoHD has no problem to reach the maximum tx power, finally solve it by changing region to Australia.

Got a few UAP LR but the old one b/g/n

try changes as suggested by heidarren
just for your info , Delivery Traffic Indication Message (DTIM) need to set to 3 or greater for apple devices, can read Here

for info changing value
DTIM

What client devices / phones r u using?

Your devices are connected at 802.11n not 802.11ac, max speed is 300mbps.