Ever since I've installed and setup my router, I've had problems when opening various websites. When a website is opened, I get an error that says something like:

"The website is unavaialable. Check if there are no errors in the website address. If the address is correct, run Windows Diagnostics"

This is then followed by "DNS_PROBE_FINISHED_NXDOMAIN"

Some very important observations:

1) this error appears usually when I'm redirected (via link) to another website or when I open a new tab;

2) this error appears very randomly and there are no pre-defined websites where this always appears (I cannot readily replicate this, as it seems to happen randomly);

3) usually the "DNS_PROBE_FINISHED_NXDOMAIN" appears for a couple of seconds and then the website just opens correctly by itself; other times I have to refresh a website for it to open correctly;

4) this error very often happens when redirected to a payment service or online banking (for example when shopping for goods or tickets) - in these cases, if the "DNS_PROBE_FINISHED_NXDOMAIN" error appears, I can only proceed by refreshing the site (waiting does not help here) and this usually causes a loss of all my payment data and I have to restart the buying/reservation process all over with the ecommerce website;

5) very often websites open with glitches (like graphics don't load, or only disorganized text is visible) and only F5 fixes it (sometimes a couple of F5s are needed);

6) all these errors only happen when I'm connected to the router - when i switch to my ISP-provided modem WiFi, the problems never appear;

7) the type of browser doesn't matter here - tried a number of them and the issue is the same. Funnily enough these errors only happen on PC and not on Android...

Today I updated to the latest firmware, but the issue persists. My router model is R8000 X6 AC3200 Tri-Band WiFi.

I've read in other posts that downgrading the router firmware back a couple of versions seems to resolve the issue. Can anyone let me know how I can go about this step by step and which version to donwload and where from? 

Thanks!