@KokeshKokesh 

I faintly remember I have seen this before on my router, which is a different TP-Link model, though. I think it was something to do with how the process was started. On my model the certificate can be created by either clicking the button "Generate" or by checking "Enable VPN Server", which will ask you to generate a certificate and only one of the two ways may have worked.

Also, if you use DDNS, then check that it is setup correctly, because the DDNS address may actually be part of the certificate.

@woozle I had it running foe a.year.or so. So the certificate was there. Now I can't save settings, all rhe buttons are diaabled.

@KokeshKokesh SOLVED! 

In Chrome select Inspect Element on Generate Button.

In Developer console in the button HTML rewrite disabled in the following to anything else and quickly change the now enabled Generate button (quickly before it redisables)

<button type="submit" class="green T_c_genCert l_auto pure-button tp-btn-custom" id="genCert" disabled><span class="undefined">Generate</span></button>

I've scrolled the page above the console, so I see the button. Was quick and made it :)