@Bungle 

Hi, Thank you very much for your feedback.

I do not have any other suggestions for your current issue and I hope you could help me check the following details so that I could forward your case to the senior engineers for further assistance:

1. Please check the IPV4 address on Deco X73-DSL to make sure it is a public IP: Deco APP>More>Advanced>IPV4.

--If you do remember the internet IP on the old router, please also check whether they are the same IP address.

2. Have you tried to open port 443 for the Synology server?

Thanks a lot.

Best regards.