Sign on the dotted line that the installer is not responsible for any issues that may occur from port forwarding?
In reality the customer will rarely pay for a high end router. So you are left with a crap ISP one, ISP's will not support anything that is not there own to end users. Then most alarm companies don't want to take responsibility for service of the router.
It's a bit of a tricky situation TBH, on a commercial site you can leave the responsibility to an IT dept or install your own service for your kit. Domestic is very different when consumers needs at a cost overrule security.
I had a customer specifically ask for LAN use only the other day, due to not trusting remote access. I don't blame him but this is a first for me, most have no idea.