It depends on how your Synology is configured. Your system can be configured to trust the Synology’s untrusted certificate.
My Synology can be reached via https://[IP ADRESS]:5001. 5001 is the standard port for local encrypted access to a Synology NAS. This can be changed manually. Accessing the Synology via the wrong port using SSL will fail.
When this page is being accessed for the first time, Safari will prompt you with (everything is translated from German, so it might be slightly different in English):
This connection is not private.
This website potentially pretends to be [IP ADRESS] in order to steal your personal or financial information. Return to the previous page.
Beneath this message you can find this information:
If you are aware of the risks, you can open this website.
You can click on “open this website”.
Now a new message occurs:
Do you really want to open this website over a non-private connection?
You can click on “visit website”.
Now you get a new prompt:
You are changing the system settings for trusted certificates. (…) Enter your password".
After entering your password, you can click on “Update settings”.
After that, you will always be able to visit https://[IP ADRESS]:5001 without further warnings.
A different approach would be to access your Synology from outside of your network. There are several options. The one I use is Quick Connect. Quick Connect uses validated LetsEncrypt certificates to provide access to your NAS and it can be set up easily. The disadvantage: this is a way to access your Synology from the internet. I need this access from everywhere, so I am fine with that. Just make sure that your Synology is secure if you enable internet access. You can check your NAS’ security using the Security Advisor. I recommend following their recommended actions if possible and you will have quite a secure experience using the NAS from anywhere.