Hi Patrick, Not quite understanding what you really want. As you said, WDS is not a option. If so, you may connect the TP AC1350 C58 router to the main router FritzBox 7580 by cable. If cable is not...

except the channel, you may also try channel width, or change mode to 802.11n only. Maybe change transmit power to low or middle

Hi, you can upgrade it without issue as the orinal one is also UN version.

as I know, google is https website, this model can't limit https webiste. btw, you may also think about access control function

Odd, what if you try different browsers or try on smart phones?

It sounds odd. Maybe you can contact the TP-LINK support for help.