I'm double NAT here. Broadband refused to play ball with a DIY router, so the ISP one hands out a DHCP lease to a single machine and everything is on the other

I'm double NAT here. Broadband refused to play ball with a DIY router, so the ISP one hands out a DHCP lease to a single machine and everything is on the other side. I'm told double NAT is very bad but overall it seems to work in practice.