Netgear R7000 Wireless AC1900 Dual Band Gigabit Router / WiFi drop outs (LAN is fine)

Dear Forum,

I have my router since 2016 and it used to work perfectly. However, in the past we had a few problems with wifi.

(1) My wife's smartphone (and my son's laptop) suddently weren't able to get online. She was connected to the router, but had an explanation mark next to her wifi-symbol: connected, but no internet. This happened out of a sudden and I was convinced it was her phone's fault. After a week or so, I did a factory reset on the router and then her phone worked again.

(2) We now have similar problems with various devices, in particular my son's laptop and playstation, sometimes also our smart tv. Sometimes we have the "connected, but no internet" explanation mark, sometimes the device cannot connect to the router even though it's 2m next of it. However, during all that time, my desktop computer (which is connected to the router via ethernet) has no problems. My mobile phone (recent Samsung S10) also works fine in general.

I just wonder what could cause these sporadic wifi issues: obviously, internet works, because the desktop computer is connected. Moreover, all the devices I have listed above used to work fine for a long time. Issue (1) happened (and was resolved) in the beginning of 2020. Issue (2) started a couple of weeks ago, and still is persistent. 

Do you have any ideas of what could be the source of this problem? I should mention that everything worked well a couple of months ago.

To be fair, I should mention the following: our internet provider has flagged some instability issues in the area in the past week. Now I'd be tempted to think this is the source of all problems. However, this wouldn't explain issue (1) which only affected two devices. It also doesn't explain why the desktop computer isn't affected. The only explanation I could come up with is that the internet stability briefly interrupts the wifi and then the devices cannot quickly connect back? (But then, as said, sometimes the devices ARE connected to the router, but simply don't get internet -- wheres other wifi devices / ethernet computer are still working well.)

Any help is highly appreciated -- would be good to understand what's going on.

Cheers,

Heiko

PS: Of course I've gone through the whole router set-up, checked access control etc etc... All seems fine -- in particular, things used to work well for all devices.