Even after creating /etc/wsl.conf
[network]
generateResolvConf = false
And specifying desired DNS server in /etc/resolv.conf, after WSL reboot, /etc/resolv.conf reverts it’s content.
Here is how to make changes in /etc/resolv.conf make permanent:
sudo rm /etc/resolv.conf
sudo bash -c 'echo "nameserver 1.1.1.1" > /etc/resolv.conf'
sudo bash -c 'echo "[network]" > /etc/wsl.conf'
sudo bash -c 'echo "generateResolvConf = false" >> /etc/wsl.conf'
sudo chattr +i /etc/resolv.conf
chattr +i command makes file immutable so /etc/resolv.conf file cannot be modified nor deleted.
geekdudes 
chattr: Operation not supported while reading flags on /etc/resolv.conf
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Your /etc/resolv.conf is probably a symlink.Try:chattr +i “$(realpath /etc/resolv.conf)”
Or try with -f flag
sudo chattr -f +i /etc/resolv.conf
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