mga paps baka may makatulong sa inyo.
Tama ba pagkakaintindi ko na dapat per device ang assignment ng ipv6 kung naka enable sa lahat? Hindi kasi nakakakuha ng ipv6 bawat device na naka connect sa network ko. Currently ganito setup ko and naka enable yung ipv6 sa main router at AX3
Is this a multi-wan setup / load balancing? And you use your ER605 as your gateway / router? Then I think you should use your Huawei as an access point only, meaning your client devices are in the same 192.168.0.0/24 subnet, DHCP on the AX3 is disabled, and it is connected to LAN instead of WAN. So you can avoid Double-NAT and make IPv6 work, because your Huawei AX3 only gets a /64 and it cannot split /64's.
You can probably set your ER605 to assign /60 subnets though, so your ax3 can assign v6 /64 addresses if you want to separate the main router from the AX3 access point. My approach is kinda different tho, my main router is the one that does different subnets, giving sub-networks (guest and main lan) their own v4 and v6 subnets.
Anyways, assuming you have proper bridge mode (so your router can get a /56 prefix), yes you can have IPv6 in every client device you have when you have at least one ISP with IPv6 support. My setup has PLDT main and Globe secondary with no IPv6, and my clients have IPv6 despite that.
Here is my setup in detail.