Offtopic alternative setup:
Run OpenWrt on the Pi 4 , plug it to the LAN port of the Archer running stock firmware, disable DHCP on the Archer, plug your internet connection to the LAN port on the Archer as well (not WAN), visit Pi4 web UI, go to Interfaces -> Devices -> Add device and choose MAC VLAN (this is the WAN), go back to Interfaces, Add Interface and choose the newly created MAC VLAN, select DHCP, set firewall to RED zone and now your Pi is doing the routing and LAN DHCP. You can easily install AdGuard Home instead of Pi-hole. There are other ad-block OpenWrt specific apps as well that are one-click install.
Your Archer is now a dumb AP, make sure to change it's LAN IP to avoid conflict with the Pi.
MAC VLAN will limit you to ~500/500 up/down since you're using one link, performance should be better too since the Pi is a bit overkill for non-VPN usage.