The short answer is, there is no way to change the MAC address without buying a separate modem and router. What you are failing to understand is that this goes beyond tying an IP address to a MAC address.

The WAN IP address will change. In modem mode, the VM DHCP servers effectively 'see' the MAC address of the downstream device attached to it, in your case the pfSense router - change the NIC and you change the MAC address and hence get a new IP address. Wireless setup not working - add MAC address to router I have a new router and the printer will not connect to the internet. The wireless network test results say that "If MAC address filtering is enabled, and you would like to keep it enabled, you must add the MAC address to your printer to the list of permitted devices for your wireless router before continuing." How do I add MAC address to router? | AT&T Community Forums

The MAC Filter feature of your Linksys Smart Wi-Fi Router is a wireless security option that will allow or prevent access to your wireless network aside from setting up a password. If you have multiple routers, the MAC addresses saved on the first router should be manually configured and saved on the succeeding routers for all to have the same restrictions as the first router.

Nov 12, 2018 2 Ways to Change MAC Address on Windows 10 / 8 / 7 Nov 11, 2017

How to Change MAC Address in Windows 10 [Easy Steps]

Some ISP may register the MAC address of your computer when you dialing up to internet for the first time via modem. If you add a new router into your network to share your internet connection, the modem may still register the MAC address of your computer, in that case, it's needed to configure MAC Address Clone on the router. How to find MAC Address of WiFi Router | Trickvilla