Hello,
First of all, if you have sent funds to an address generated from your wallet backend, the funds will be in that wallet, even if the plugin does not show this.
I have never come across the particular issue you describe. You say that after installing the plugin, and even before activating it, you see error 504 Gateway Timeout.
Normally timeout errors can occur when the plugin attempts to contact the RPC API of a full node wallet, such as Bitcoin core, but there is a firewall in between, blocking communication. However, since as you say, the plugin is not active, a firewall cannot be the problem. Please tell me the following:
1. Can you confirm again, that after installing (not activating) the plugin, your site stops, and after you remove the plugin files the site works again? I am asking you to check again because this is very unusual. Recent versions of WordPress can recover even from a plugin that crashes, by deactivating the plugin.
2. While the plugin was still working, which coin adapters were you using?
3. Was the built in Bitcoin core node adapter enabled or disabled?
4. Which version of PHP are you running?
5. Can you check the PHP error log on your server? There will be more information there about your problem and what causes it.
Looking forward to your reply.
with regards