You could plug in an USB drive and use it as an external storage to install Adguard.
Be aware: If you use the manual installation and want to install Adguard in a different directory you have to change the install script like this:
#grab and install AGH and install it in a custom directory (/mnt/sda3/opt/adguard)
curl -s -S -L https://raw.githubusercontent.com/AdguardTeam/AdGuardHome/master/scripts/install.sh | sh -s -- -c edge -o /mnt/sda3/opt/adguard
Where /mnt/sda3/opt/adguard is the directory you want to install Adguard to.