I’m in the same situation as you — my ISP only provides IPv4 through CGNAT, but I do receive a public /56 IPv6 prefix, which I actively use.
1. Web Interface Access over IPv6
You can access Keenetic's web interface remotely via IPv6, but only if HTTP is enabled.
HTTPS does not work at all, because Keenetic does not have a valid SSL certificate installed by default. That’s why HTTPS connections are rejected by browsers.
To enable HTTP access:
Go to Users & Access > Management Access in the Keenetic web interface.
Enable the "Remote web access" option.
Make sure only HTTP is selected (HTTPS won’t work without a valid certificate).
Copy the device's IPv6 address from the interface.
Access it in your browser like this:
http://[your_ipv6_address]
2. Telnet and SSH Access over IPv6
Telnet works over IPv6. You can connect to Keenetic using a Telnet client and the device’s global IPv6 address.
SSH does not work over IPv6, even if it’s enabled in the web interface.
This is likely because Keenetic does not support SSH over IPv6 at all (at least in current firmware versions).
ru.celebi's post in Access to router over IPv6 (Web, SSH) was marked as the answer