@STN Never changed SSID, still using the same. IMO - you are righ, probably any specific sequence in settiong is responsible for this problem. But the question is: which sequence? :)

@STN I switched language again to Polish (as per tp-link support request) and now everything works fine. I don't know how is it possible... I asked engineer if he can help you. Will see what will he...

@STN Few days ago, I did factory reset on my router, because I had some additional problems. Before reset, backed up of course my configuration, and restored after reset. Unfortunatelly nothing...

@STN Hmm, very strange...

@STN Wireless Settings: Guest Network: Disabled Wireless Schedule: Disabled WPS: Disabled Additional Settings: And my sequence in which I changed language: 1. Access Control: Disabled, Language:...

I have found solution/workaround. Changed language from polish to english o router settings and now everything works fine :) In few cases, I had to remove and add again device to whitelist after...