Hello,
We’ve had a similar issue here: RMS + EE UK Connection Issue
From what I was able to find online, EE uses 464XLAT for transitional period from IPv4 to IPv6. This way of providing both addresses is not very common, thus our devices do not support it, and instead it only obtains either IPv4 or IPv6 address.
While we do not have 464XLAT implementation, it can be downloaded from opkg feeds. It should be noted, that this is not officially supported, so you may experience side-effects. Additionally, all of the configuration will need to be made using SSH/CLI.
Best regards,