Best WIFI configuration for HG8145V5 (range configuration)?
Solution 1:
The only thing you can do with this to improve the range is raise TX power to max available. That may not be enough, because Wi-Fi is a shared, half-duplex medium.
To clarify, a single client connected to the AP has the entire bandwith for itself. When the second client connects, both of them get a half. When the third connects, they all get a third. And so on. In practice there are mechanisms to alleviate the situation, but the principle holds. A g-radio has max bandwidth of 54Mb/s. Divide that for 20 clients, and add the fact that in real life a client can achieve max about 2/3 of the advertised, theoretical transfer rates...
The further away a client is from the antenna, the lower speed it can transmit with. Walls and other obstacles between the client and antenna also drop the available transfer rate. One single slow client will affect the network performance for all clients. 802.11b max throughput is 11Mb/s, so adding b to the soup would allow 802.11b -clients (very old by now) use the network, which would just drag down the performance.
Other than that, you're operating on the 2.4GHz range which has been notoriously congested for a decade and a half. What you can do is download inSSIDer and use it to see what other devices are operating on 2.4GHz range, and set the AP channel to whatever has the least amount of other devices. However due to channel overlap there are only 3 usable channels - 1, 6 and 11 in Americas, 1, 3 and 17 in EU and most of the rest of the world.
Alternatively you can use channel exclusion list to deny usage of all other but those 3 channels to prevent co-channel interference. Auto channel will allow the AP to swap to another channel when there's too much interference on the channel it's running. This'd be my configuration of choice if 2.4GHz is required.
If the AP supports wide channels on 2.4GHz, you could also try that to improve the available bandwidth. Wide channels on 2.4GHz isn't generally recommended for variety of reasons, but at the end it's all depending on the site. If you use wide auto channel with channel exclusion list, you should exclude all others but the very first and last one.
This is an n-device, which means it's also capable of supporting clients on n-channels available in 5GHz spectrum. You should allow it broadcast on both spectrums. A likely consequence is that n-capable devices closer to the AP will hook up with the 5GHz radio, whereas devices further away might go to 2.4GHz. Even then their connectivity should improve as there's now less competition for the bandwidth.
If all connecting devices are also n-capable, general recommendation is to drop 2.4GHz spectrum altogether and use only 5GHz. This is because 5Ghz spectrum offers much higher bandwidth, is much less congested and wide channels aren't a problem.
As davidgo commented, 802.11n also offers double the range of 802.11g. Here's an old Computerworld article discussing the usage of then-new (2010) n-technology.
That said, this is only general advice.
Due to the nature of Wi-Fi there is no "best configuration", only best configuration that can be created for that specific location. Which means that in the long run situation can be improved online only to a certain extent. It's possible that you will need onsite assistance from a person knowlegeable in Wi-Fi deployments.