2.4 vs 5

This is likely to be a basic question for many and so please bear with me.

I understand that 2.4GHz signal has a longer range and better ability to penetrate solid wall, compared with 5GHz signal.

My RT-AC87U sits in room A, sending out both 2,4GHz and 5GHz signals. In room B, I find that the 5GHz signal is stronger than the 2.4GHz one. In a straight line, the router and client is about 10-12 metres apart. And there are 2 concrete walls in between.

This surprises me as I expected the 2.4GHz signal would be stronger in room B. So, what might be the reason(s) for this?

Could it be the 2.4GHz band is having a lot of interference from other 2.4GHz networks of my neighbours? Could this kind of congestion degrade the signal quality?

If this could be, then what router setting(s) can be adjusted to improve?
Currently, the settings are default settings as per Merlin's 378.55.
Ie
Channel bandwidth = 20/40 MHz ;
Control Channel = auto

I imagine 'auto' channel will have the router to choose automatically the best channel to avoid interference and congestion? If this is the case, in what scenario would a manual choice of channel number would bring a benefit?

I am a novice in this and appreciate any help.
Thank you.

The best way to determine channels are to use a software like inSSIDer or wifi analyser app. Another thing you can do is do a few scans from the router and see the channels least use. Channels like channel 13 are never used on auto so if it is legal where you are it is an option. Some devices may not see channel 13 but can actually connect to it.

Stronger or faster? Those are not the same thing. In my experience 11ac 5GHz tends to be a lot faster to at least mildly faster than the 2.4GHz band right up until 5GHz performance drops off a cliff.

Example, in my house at close range I can hit as much as 60MB/sec downloads from my router to my laptop on 5GHz and 28MB/sec on 2.4GHz. A couple of rooms and about 30ft of distance and 5GHz is down to 20MB/sec and 2.4GHz is down to 16MB/sec. Move another 10ft and one more room and 5GHz suddenly plummets to 3-5MB/sec but 2.4GHz is at 11MB/sec.

With my old 11n, it would have been 25MB/sec 2.4GHz and 5GHz close, about 7MB/sec 5GHz at the second position and 12MB/sec 2.4GHz. In the final position it would be no usable connection on 5GHz (I could connect and stuff was transfer in little spurts, but web page loads would take forever, often hang and a file transfer would do 300-700KB/sec and frequently drop off to nothing for 10-15s at a time) and 8MB/sec on 2.4GHz.

Of course with the old 11n router or the newer 11ac router, 2.4GHz would just keep going. I could move a room further and still get 2.5MB/sec or 3.5MB/sec (respectively) on 2.4GHz. I could still see 5GHz, but I couldn't connect at all. (total distance about 60ft, a floor and 3 walls).

The said client is a Netgear EX7000 extender.

I set it up in room B. The band icon has 3 colours to show the connection: red for poor, amber for good, green for optimum.
For the 2.4GHz band, its icon shows amber. For the 5GHz band, its icon shows green.
If I move the EX7000 around, the 5GHz band icon remains either green and sometimes amber while the 2.4GHz band icon shows amber and sometimes even red.

So, in general, it seems the EX7000 receives the 5GHz signal better (ie stronger).

I have just installed the Netgear app (WiFi Analytics) in my smartphone. Is this a reliable app to provide more-or-less accurate measurements?

I wouldn't bet on it actually being stronger. How Netgear has decided to represent the signal strength likely has no correlation with actual signal strength.

Where I am getting 20MB/sec on 5GHz and 16MB/sec on 2.4GHz, my 5GHz signal strength is about -74dBm and my 2.4GHz signal strength is -62dBm.

The 2.4GHz signal strength is significantly stronger (since log scale, that works out to 16x stronger signal strength), but it is still slower. 40MHz wide channel vs 80MHz channel, the weaker 80MHz channel still wins in this case.

Probably Netflix is simply implementing a modulation rate "signal strength" indicator. If it is connected at a higher modulation rate, the signal strength is higher. In the end, that is really all that matters for the end user, even if the signal isn't actually stronger.