The ER7212PC can provide nice fail-over for IPv4 via NAT using multi-WAN setup, but for IPv6 it's not working, as only one of the WANs can be set to provide IPv6 connection to the LAN.

Other devices like ER605 and OC200 controllers have same limitation. Some Feature Request (https://community.tp-link.com/en/business/forum/topic/603704) was published in 2023 but yet without proper attention.