Guide for less problematic setup?

[deHakkelaar]

@123

As soon as I set the DNS on the router back to Google (8.8.8.8)

Can you post a screenshot of what setting you changed back ?
I suspect your setting DNS server for the WAN part on the router.
Your not supposed to do it like that as you miss out on very nice extra's.
I suggest you default those DNS settings on the router, disable the LAN DHCP server/service on the router, and as a replacement, activate the DHCP service from Pi-hole (via the web GUI):

How do I use Pi-hole's built in DHCP server (and why would I want to)?

Using Your Existing Router For Network-wide Ad Blocking You might not need to use Pi-hole's DHCP server: In many home environments, your router also functions as your DHCP server. In this case, you can often set Pi-hole to be the DNS server for your network clients in the router's DHCP (or LAN) settings page, which allows all of your network clients to block ads simply by connecting to the network. Setting it up this way is also what makes Pi-hole very powerful for network-wide ad blocking. W…

This is reply on one of my clients with 10.0.0.2 being my Pi-hole:

$ nslookup pi.hole
Server:    10.0.0.2
Address 1: 10.0.0.2 noads.dehakkelaar.nl

Name:      pi.hole
Address 1: 10.0.0.2 noads.dehakkelaar.nl

And below one should display a nice block page:

http://doubleclick.com

EDIT: Owh and dont forget to renew DHCP leases on the clients when you switch by disconnecting & reconnecting network on the clients or reboot them!

Ps, your right, its a maze of settings with all the different hardware and different setups that people have but YOU try to write a manual that covers it all