Before my old router died, my NAS could accept incoming connections for any of its global (self-assigned) IPv6 addresses. After switching to my ISP's modem/router, incoming IPv6 connections no longer work.
The new router automatically assigns every device an IPv6 address of the form 64-bit-prefix::n, where n is an incremented integer.
Using the "Connected Clients" feature of my router (to show connected clients and their IP address(es) I can see all of my other IPv6-enabled devices with their router-assigned addresses (IPv4 and IPv6), and their self-assigned IPv6 address(es), except for my NAS (it only shows the two router-assigned addresses).
I've locally verified that the self-assigned IPv6 addresses are "assigned" on the NAS, and that "IPv6 Simple Security" is disabled on the router.
What's going on? Obviously my other devices are advertising their self-assigned IPv6 addresses just fine, and it used to work on the NAS without any additional configuration.
My NAS is running Linux/Ubuntu 19.04.