If you have access to the ISP's router, you can try to create a NAT for port 500, but I've personally tested it and it didn't work.
So what I can say is that it probably won't work.
My suggestion is that you hire a dedicated link.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
Please, if this post was useful, leave your kudos and mark it as solved.