Error getting public keys

Question

When I execute some apt command like install or update, it produces multiple errors of public keys not found and it can't finish executing the install or update. For example, this is one of the multiple same errors:

W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://mega.nz/linux/MEGAsync/xUbuntu_20.04 ./ InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 03C3AD3A7F068E5D
W: Failed to fetch https://packages.cloud.google.com/apt/dists/cloud-sdk/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY FEEA9169307EA071 NO_PUBKEY 8B57C5C2836F4BEB

I've already tried with

sudo apt-key adv –keyserver keyserver.ubuntu.com –recv-keys [key]

for each key, but every key gives me one of two errors:

gpg: WARNING: no command supplied.  Trying to guess what you mean ...
usage: gpg [options] [filename]

and

gpg: keyserver receive failed: No name

if I execute

gpg --keyserver wwwkeys.eu.pgp.net --recv-keys [key]

I also can't install PPA Manager because every tutorial implies executing

sudo apt-get update
sudo apt-get install

which gives the errors. I also tried executing

sudo rm /etc/apt/trusted.gpg

before getting the keys.

I've also tried

PUBKEY=[key]
gpg --keyserver subkeys.pgp.net --recv-key $PUBKEY && gpg -a --export $PUBKEY | 
sudo apt-key add --

And I get

gpg: keyserver receive failed: Connection timed out

I have Ubuntu 20.