Network Manager doesn't respect DHCP settings?

Question

Had my Ubuntu 20.04 system running without any issues for months, but lately I had to start running sudo dhclient after every reboot, otherwise I am not able to connect to the internet.

I am fairly sure this issue has to do with my recent attempt at trying out Wireguard which I may have misconfigured resulting in a broken setting somewhere (Wireguard is now uninstalled).

Right after login, my network manager connection looks like this, which was identical when everything was running fine:

After running sudo dhclient, I have network but the network-manager settings haven't changed.

I could add the dhclient command to a startup script, but what I would really like is to understand what got messed up!

Here is my /etc/netplan/01-network-manager-all.yaml config

# Let NetworkManager manage all devices on this system
network:
  version: 2
  renderer: NetworkManager

Thank you in advance for your help!

Edit: more details

I am connected to my router both through ethernet and wifi, and on startup only ethernet seems to have issues. My DHCP client is a pihole docker container running on a NAS on the LAN. My setup didn't change recently tho.

running ifconfig, these are the outputs for my enp4s0 and wlp5s0 interfaces:

enp4s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.1.108  netmask 255.255.255.0  broadcast 192.168.1.255
        inet6 fe80::e8fc:b999:680:6d7d  prefixlen 64  scopeid 0x20<link>
        ether 0c:9d:92:8a:c9:7a  txqueuelen 1000  (Ethernet)
        RX packets 631  bytes 563240 (563.2 KB)
        RX errors 0  dropped 1  overruns 0  frame 0
        TX packets 578  bytes 65883 (65.8 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
        device memory 0xfc400000-fc41ffff
enp4s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.1.108  netmask 255.255.255.0  broadcast 192.168.1.255
        inet6 fe80::e8fc:b999:680:6d7d  prefixlen 64  scopeid 0x20<link>
        ether 0c:9d:92:8a:c9:7a  txqueuelen 1000  (Ethernet)
        RX packets 631  bytes 563240 (563.2 KB)
        RX errors 0  dropped 1  overruns 0  frame 0
        TX packets 578  bytes 65883 (65.8 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
        device memory 0xfc400000-fc41ffff

the output doesn't seem to change after running sudo dhclient, but I noticed one RX dropped error, and only on the ethernet interface.

Edit 2:

Looks like my /etc/resolv.conf file doesn't exist at startup (may be an obvious thing but better to add this info

before:

cat /etc/resolv.conf

after:

cat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
# 127.0.0.53 is the systemd-resolved stub resolver.
# run "systemd-resolve --status" to see details about the actual nameservers.
nameserver 192.168.1.112

Edit 3:

my /etc/systemd/network folder is empty

running sudo iptables -L before and after running dhclient shows the following differences (only differences shown since I use PIA VPN and it adds a ton of entries, but it has been installed on my system for months too):

before:

Chain piavpn.r.300.allowLAN (1 references)
ACCEPT     all  --  anywhere             224.0.0.0/4
[...]
This stuff below isn't present after running the command
Chain ufw-skip-to-policy-output (0 references)
target     prot opt source               destination

ACCEPT     all  --  anywhere             anywhere

Chain ufw-track-forward (1 references)
target     prot opt source               destination

Chain ufw-track-input (1 references)
target     prot opt source               destination

Chain ufw-track-output (1 references)
target     prot opt source               destination

ACCEPT     tcp  --  anywhere             anywhere             ctstate NEW
ACCEPT     udp  --  anywhere             anywhere             ctstate NEW
Chain ufw-user-forward (1 references)
target     prot opt source               destination

Chain ufw-user-input (1 references)
target     prot opt source               destination

Chain ufw-user-limit (0 references)
target     prot opt source               destination

LOG        all  --  anywhere             anywhere             limit: avg 3/min burst 5 LOG level warning prefix "[UFW LIMIT BLOCK] "
REJECT     all  --  anywhere             anywhere             reject-with icmp-port-unreachable
Chain ufw-user-limit-accept (0 references)
target     prot opt source               destination

ACCEPT     all  --  anywhere             anywhere

Chain ufw-user-logging-forward (0 references)
target     prot opt source               destination

Chain ufw-user-logging-input (0 references)
target     prot opt source               destination

Chain ufw-user-logging-output (0 references)
target     prot opt source               destination

Chain ufw-user-output (1 references)
target     prot opt source               destination 

after:

ACCEPT     all  --  anywhere             base-address.mcast.net/4
[...]
entries missing

Let me know if you need other info

Answer 1

After troubleshooting all day long, I just gave up: instead of understanding what was causing this issue I just fixed the problem even though I am 100% sure the issue comes from somewhere else:

Based on this askubuntu answer I removed the symlink from /etc/resolv.conf and added my own DNS entry. After a restart I noticed the file got replaced with one created by Network Manager, so I just updated that one.

Everything is back to normal, even though I am still unsure what caused all this mess.