The router does have a firewall but it blocks everything inbound by default. Some routers (at least mine) do not offer the granularity to filter traffic for certain devices (no NAT either). It's either allow all in or nothing.
When you enable IPv6 and switch off the firewall (since you can't host anything otherwise), every device becomes exposed to the internet.
Then unless the devices have a firewall themselves, all is exposed. Not just the web services, ssh and the rest as well.
This is correct. My router however doesn't have that level of firewall. It's either all allowed or nothing is.