What I would suggest instead of this complicated setup is to use the Time Capsule as your internet router. Use a device like this between your ISP’s service and the Time Capsule: https://www.amazon.com/Internet-Network-Security-Switch-Net/dp/B00SNW6SIA
This will allow you to turn internet service on and off to meet your needs. Turn it on when you want service, off when you don’t, and this will allow you to connect with other devices on the internal LAN (such as the Time Capsule for backup and the printer) even when you have the internet service turned off. And that will allow you to connect the Mac to one of the ethernet ports on the Time Capsule.
It is rare that an ISP will provide more than a single IP address to an end user, so that is why you cannot connect their incoming service to a switch or hub and split the connection - only one of the devices will get the IP address from the ISP.