I'm no expert but this seems to work for me.
# Install ee-acme from VirtuBox
bash <(wget -qO - https://raw.githubusercontent.com/VirtuBox/ee-acme-sh/master/install.sh)
# dnsutils so dig can resolve correctly (could already be installed for 18.04?)
apt install dnsutils
# copy and rename ee-acme.sh
cp /root/.ee-acme/ee-acme.sh /usr/local/bin/wo-acme
# replace outdated ssl ciphers /etc/nginx/nginx.conf in line 45
ssl_ciphers "EECDH+AESGCM:EDH+AESGCM:AES256+EECDH:AES256+EDH";
# restart nginx
service nginx reload