Hmm,
5 euros per month for a static IP sounds very reasonable, but I’ve never heard of a 100 euro set up fee; that sounds like a lot.
Are you sure that you cannot find an ISP in your area that offers static IP with 5 or 10 euros per month but without an upfront fee?
In any case, it is possible to make this work even with a dynamic IP, but it’s somewhat harder, and it will involve connecting to your WordPress machine via ssh.
If you can get the IP of the server via a Linux command, you can use wp-cli to insert it into your WordPress DB. Let’s say that you have a command that pulls the IP from somewhere, then you can do: wp option update wallets-bitcoin-core-node-settings-rpc-ip $(command that pulls your ip) --path=/path/to/wordpress
Just replace “command that pulls your ip” with the actual command and replace /path/to/wordpress with your actual path.
The difficulty here is to have a command that pulls the dynamic IP. Since your web server already has a static IP, you’d have to push your dynamic IP FROM your wallet server TO your WordPress server. For example:
Wallet server:
curl https://api.ipify.org >>myip.txt
scp myip.txt wordpress:~
WordPress server:
wp option update wallets-bitcoin-core-node-settings-rpc-ip $(cat ~/myip.txt) --path=/path/to/wordpress
Do this once manually to see that it works. Then, on both ends these should be on the crontabs of the two machines. This way the setting will get updated shortly after your dynamic IP changes. There might be up to a minute of downtime when the IP changes.
Let me know if you are having trouble implementing this.