Pi-hole comes with its own "network manager" called dhcpcd5:
pi@noads:~ $ apt policy dhcpcd5
dhcpcd5:
Installed: 1:6.11.5-1+rpt7
[..]
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
[..]
If use network-manager to configure your network, make sure its "active":
sudo service network-manager status
Check if dhcpcd is also active which means both network managers are fighting over control:
sudo service dhcpcd status
Disable dhcpcd if active:
sudo systemctl disable dhcpcd
Reboot and check connectivity:
sudo reboot
journalctl -u networking
journalctl -u network-manager
ip l
ip a
ip r
You could remove/uninstall dhcpcd5 if have no use for it:
sudo apt-get remove dhcpcd5