WNDR3400v3 connects 5G and ethernet but not using 2.4G

1) Yes, quite a few times, usually using the power switch, sometimes unplugging the power cord. I didn't get the original access point to work with reset only. Another access point seemed to work for a short while, but stopped working quite soon.

2) Yes, but only once. Immediately afterwards the original access point worked (or seemed to) for a while, but stopped working afterwards. I cannot be absolutely sure if the 2.4G access points really worked, I relied on the notification of the tablets (e.g. connected, no internet). And when the connection to internet was lost, it was too late to ensure.

3) I already have the latest firmware, so I cannot upgrade it.

4) I have another ADSL router in use at the moment, but it has different GUI and features (and problems), so having the original NETGEAR back would be nice... I will surely try it later again when I have the time.

Another symptom that I forgot to mention was that even though it showed that ADSL was connected, and I was able to access the GUI, all the computers in the network reported that they lost access to internet. Intranet was working, and GUI showed 20M+ DL and 1M+ UL speed towards ISP, there was no connection to internet from any device connected to ADSL router. After restart the connection to internet was resumed. It started to happen more and more often and it continued to happen after reset to defaults as well.

I had no USB devices connected there, only 4x eth cables (printer, NAS,8 port switch, computer). I also tried with only 1 computer connected via ethernet (+ the some devices using wifi on 5G band).