The reason I want to use a VPN is to prevent intruders from access to my personal data, especially financial data, and to protect my devices from attacks.
Above, I put in a post describing my system, I deleted it as others were describing answers for a different requirement, I thought I'd got the wrong end of the stick. So I will describe it here, because it does seem to do what you appear to require. It also connects to my web server, not relevant to you, but that is part of my system.
My internet connection is via my phone line, this is connected to an ADSL router supplied by my internet provider. This has four Ethernet ports and built-in WiFi. I don't use the ADSL router's WiFi (other than for testing).
An Ethernet port from the ADSL router is connected to what I call an intelligent router. This is then connected by other Ethernet ports to my web server, another to my Mac, and two more ports to two separate WiFi routers (each serving different areas of the house). Logically, the server port is on its own network, the other ports are logically all on the same house intranet network.
The intelligent router is programmed so that external requests can only go to my server, but not to the intranet ports, but any device on the intranet can make any external access request. No device on my intranet needs a firewall, the intelligent router takes care of that (though anti-virus software is still necessary, of course).
However, I will point out that this is a fairly expensive system (all that "extra" hardware), and programming the intelligent router took my IT-savvy son two or three hours (and many mugs of tea). But it works perfectly, doesn't slow any external access, and currently I have two desktop Macs, one Mac laptop, television, my phone, an iPad, and (until Covid-19) several visitors' devices. The only way anyone could conceivably access my intranet is if they guessed the password to WiFi that (obviously) leaks out of the house.
For completeness of this description, the intelligent router can steer requests directly from my intranet to my web server, so I can access it for updates or whatever with no external "leakage". There are other "clevernesses" associated with the server, but irrelevant here.
Hope that's understandable, whether or not it's of any help ...