You will see in my original post that the router already has 174.
If I give it 169 as well I am left with 4.

That's how public subnetting works though, I've found that it is possible to use the 'network' address i.e. x.x.x.168 with some networks but it's not recommended. You always lose that last address to the 'broadcast' IP.

So anyway, you can use any of those addresses as the router's IP on that public subnet, so .174 or .169 will be fine - you can use the same fixed IP address on the router as the 2nd subnet IP so you don't have to waste two IPs on the router unless you maybe need to for some reason.

I have a dim memory of a post on this forum ages ago which suggested that the router was actually happier with 2 of the public IP addresses - one for its WAN duties and a different one for its 2nd subnet gateway role. I vaguely remember the advisor saying that the Vigor could handle both tasks with a single address but it wasn't advisable. Pity I didn't pay more attention at the time.