Orbi 770, ipv6, Gigaclear - works then drops

I'm trying to get my Orbi 770 to have a stable ipv6 connection with my ISP Gigaclear. Initially it works, then it doesn't.

The Orbi 770 is running Firmware Version V10.5.20.10_2.2.51

My connection to Gigaclear is via a new DKT Comega ONT. I have a static IPv4 address from Gigaclear.

On the Orbi web UI, I have these IPv6 settings:

Internet Connection Type: Auto Config

DHCP User Class: blank

DHCP Domain Name: blank

Router's IPv6 Address On WAN: Not Available

Use These DNS Servers:

Primary DNS

2606:4700:4700:0000:0000:0000:0000:1111 (Cloudflare 1.1.1.1)

Secondary DNS

2620:00FE:0000:0000:0000:0000:0000:00FE (9.9.9.9)

LAN Setup

Router's IPv6 Address On LAN

2a06:61c0: etc etc

IP Address Assignment: Auto Config

Enable RIPng (off)

Use This Interface ID: blank

IPv6 Filtering: Secured

My Windows 11 PC, connected via ethernet to one of the Orbi Satellites

1. Gets a proper IPv6 address

2. Scores 10/10 on https://test-ipv6.com/

My Samsung S25, connecting via wifi 7 or 6 or 5

1. Gets a proper IPv6 address

2. Scores 10/10 on https://test-ipv6.com/

Even my Linux VM scores 10/10.

But then after some hours (more than 4 less than 24), the IPv6 fails and the https://test-ipv6.com/ scores become 0/10 on all devices.

I've spoken to Gigaclear. The person on the phone was not aware of any special settings or issues. They are passing this to network engineers.

Can this be fixed?

I've tried:

1. Turning on / off everything

2. Changing

Internet Connection Type: Auto Config

DHCP User Class: blank

to

Internet Connection Type: DHCP

DHCP User Class: 1

For a while it works, then it doesn't.