Pi-hole comes with dhcpcd5 for network settings and applying them:
pi@noads:~ $ apt show dhcpcd5
[..]
Description: DHCPv4, IPv6RA and DHCPv6 client with IPv4LL support
dhcpcd is a one stop network management daemon which includes
* RFC compliant DHCPv4 and DHCPv6 clients
* DHCPv6 Prefix Delegation support
* IPv4LL (aka ZeroConf) support
* ARP address conflict resolution
* Link carrier detection
* Wireless SSID profiles
* ARP ping profiles
You can check if its still installed with below one:
pi@noads:~ $ apt policy dhcpcd5
dhcpcd5:
Installed: 1:6.11.5-1+rpt7
Candidate: 1:6.11.5-1+rpt7
[..]
You could try remove the package with below one and hope network settings revert back to your distro's default:
sudo apt remove dhcpcd5