Is my business class isp selling me a VPN service instead of a genuine statically assigned ip address?

I am an it guy for a number of small businesses in my local town. Its not enterprise level stuff that i deal with, fairly basic poibt of sale systems, alarms, basic networking, etc.

At any rate. My question isnt avout the scope of what i do, its about what the ISP does. Carrier grade stuff is closer to enterprise grade stuff, dog.

So we are buying a statically assigned address , with built in Wi-Fi. What I've seen this isp (cable company) do in the past is install 1 modem/router (gateway) combo to handle the /30 network. when using Wi-Fi, your public address is the same as the gateway. All 4 ports on the gateway are set up to dynamically assign private addresses. You can plug anything into port 1 and get your usable ip address by assigning it statically.

The last customer i had also got the same thing from our cable TV isp. So instead of one gateway, they have a standalone modem and a standalone router. I figured the modem is just a bridge (it is, as far asi can tell. when refreshing my dhcp lease through a normal modem for dhcp internet, the terminal on my computer responds with messages from the routwr in my isp's office downtown) and through it the isp would set up the router the same way as the old setup. When i checked my public ip over the wireless network, it was very different that the /30 block they had been assigned. I asked the technician about it , and he said that they set up the internet dynamically. That was the extent of his knowledge.

So if that router is getting its address assigned by the isp's router (it is confirmed by the router's gui page) and that address is in something like a /24 subnet (i really dont remember exactly what it was, but it was a sizable subnet) and the /30 subnet i had been assigned was different than that dhcp subnet, its probably a vpn making that work, right?

Because if that router talks to the router in the isp office, with only a bridge (modem in between) the isp router should be able to assign that /30 block to my router, right? Why would they even be using a vpn then?

Like i said, my expertise is in neither enterprise grade nor carrier grade networking equipment, it is moreso in the realm of the physical layer. Im trying to learn more about routing/switching but i really dont have anybody to bounce ideas,off of. So i apologize if i come off as ignorant or misinformed.

One more clarification: this is the correct subreddit. Im not asking how that equipment is interfacing with my clients equipment. I am asking how the router installed in my client's office is interacting with the router in the isp office, over the isp network.