Hi Dan, thanks for the heads up 
I've followed this guide to solve this issue: How-To: Using Pi-Hole with Telekom Speedport routers
Sorry for the (possibly) stupid question, but is there a way I can check the IPv6 configuration myself?
I did run the pihole -d command again and found this, which seems like I configured it correctly:
I will keep you updated about the "11AM DNS issue" of course. Thanks again for your help