prepare the new server, make a new installation of the very same version of ISPConfig you have on the origin server;
    make sure you have the same php packages installed on the new server. You can generate a list on the origin server with
    dpkg --get-selections | grep ^php | awk '{print $1}' | xargs
    and paste the output to apt install on the destination;
    make a backup of the ispconfig database on origin. If you want you can change the IP (and eventually the hostname) with sed:
    sed -i 's/1\.2\.3\.4/5\.6\.7\.8/g' db-dbispconfig-2019-05-25.sql
    sed -i 's/old\.host\.name/new\.host\.name/g' db-dbispconfig-2019-05-25.sql
    And restore it on the destination with:
    cat db-dbispconfig-2019-05-25.sql | mysql -u root -p dbispconfig
    connect to the new server ispconfig panel and go to Tools > Resync, check the All services checkbox and press Start;
    rsync /var/www with:
    rsync -a --stats --progress --delete ORIGIN_SERVER:/var/www/* /var/www/
    rsync /var/vmail by adjusting the command above
    rsync /etc/letsencrypt
    rsync -a --stats --progress --delete ORIGIN_SERVER:/etc/letsencrypt/* /etc/letsencrypt/
    make a dump of all c* databases and restore them on the new server;
    if you want to retain the same ISPConfig panel certs copy /usr/local/ispconfig/interface/ssl/;
    move DNS and all related stuff to the new IP.