What types of custom things are you talking about? I'm not familiar with that type of internet connection (I've only ordered residential service.
This is not always the case, but it is the most common case.
So when a customer demands a dedicated static IP address, I wonder if they really need that. We charge extra for that because we have a limited pool of public IPv4 addresses. We can do it, but it is usually unnecessary, and better to avoid unless you really need it.
I'd assume they're restricting access further by only allowing certain ips thru a firewall in front of the VPN server, or requiring extra auth factors from unknown ips, or something.
There are other valid reasons for restricting the IP. I ask about those. I'm not arguing with the customer. I'm trying to be helpful.
Unless the customer doesn't want help.
When you have a corporate IT rep who is clueless about the options, they are not in a great position to build the optimal solution.
When you have an ISP rep who is clueless about the requirements, they are not in a great position to build the optimal solution.
When you have an ISP rep who is not willing to explain the options, you have a problem. When you have a corporate IT rep who doesn't know the requirements, is unwilling to explain the requirements, or is unwilling to learn about the options, you are unlikely to achieve the optimal solution.
When you have a helpful and capable ISP rep and a cooperative and capable IT rep, the world is a brighter place. But some levels of trust require either naivety or time and shared experiences. That's difficult when every contact between the customer and the ISP involve different people each time.
So, again, just like everything else, try to work with as small and local of an entity as possible to get the job done. This is true for government, auto dealers, ISPs, or anything else. Only scale up when there is an advantage to doing so.