Make sure you are not using a root account for that purpose. External connections for root are disabled by default in MySQL. Try using/creating a different account or modify root account permissions:
#Remote access for any IP
GRANT ALL PRIVILEGES ON *.* TO 'root'@'%' IDENTIFIED BY 'password';
#Remote access for Specific Ip
GRANT ALL PRIVILEGES ON *.* TO 'root'@'your_desire_ip' IDENTIFIED BY 'password';
Since you are running PHPMyAdmin on an external host, it is also required to check and configure your firewall to allow incoming connections on the 3306 port.
Depending on the installation, you may also need to check your MySQL configuration for server bindings since it may be equal to 127.0.0.1 by default preventing your MySQL server from listening for external requests at all.
All those changes have to be done on both MySQL hosts participating in the cluster.
Honestly, I wouldn't say I like how most SQL clusters are built and performing. That is why I prefer running them inside a single highly-available virtual machine on top of mirrored virtual storage like virtual SAN https://www.starwindsoftware.com/starwind-virtual-sanOpens a new window. That makes management, maintenance, and backup/restore much more straightforward.