The deletion was me, for my post, I wanted to read Asus RT-AX55 manual as well. I had only read Drytek at the time I first posted.
This is not easy to answer and my knowledge from limited experience of using consumer grade routers albeit fair amount of them and I may be wrong or misguided in my beliefs ...
The DrayTek Vigor300b does not support load balancing and IPv6. It supports IPv6 but only IPv4, in your local network, when you include load balancing. I am not actually sure that any consumer grade routers do IPv6 + load balancing/failover that donâ€™t cost ++++ dollars.
You could try custom firmware from https://www.asuswrt-merlin.net/ but it appears your selected Asus router isnâ€™t covered because well it's considered a *cheap router (cheap, not my words).
Which leads to this point, PFsence routers (which is computer hardware or dedicated off the shelf builds), and are not considered consumer grade, do. I actually moved to one of these from Asus routers (Asus routers cost is stupid high for what you get ~imo), but I don't have IPv6 from my ISP to answer, yes this works but I believe it will. So, at least consider looking into PFsence if this must be accomplished and you have the skills to program / use them. The learning curve is real with Pfsence, but I found, very worth it and to be honest fun, YMMV.
Otherwise my recommendation is use drytek have IPv6 facing the Internet with IPv4 for your local network, the translation layer in the drytek *should allow IPv6 to resolve locally. (*I donâ€™t own one to test and my ISP has not activated IPv6 yet)
So, in summary, IPv6 within local networking with load balancing/failover is RARE as request for consumer grade router and not sure I ever used consumer grade that can do it but they might exist.
My last point, is your ISP router, if your using in front of the consumer router (whatever you choose), the ISP router must support IPv6 handover in the mode you are running it, there are lot that donâ€™t support IPv6 when running in bridge mode and you compromise network security using ISP router to handle anything more as they often have backdoors built in.
Hope you find some this helpful in your quest.
For Pfsense, it's good but I don't know if it supports Dual stack IPv4 & IPv6 load-balancing or not and I have to find some kinds of old pc to install the FreeBSD (I recall) to it, however, I'll try with some Intel NUC I can find.
Thank you for your supports!
Try reading through here
Download PFsence (on what you coble together for a test, use Intel LAN's)
If you want decent NUC device, can I tempt you in something like this.... may have bought one (ok, yeah I did and love it)
It's expensive, for Pfsense, should be some devices I can use for 2 WANs RJ45 to load balance them, also another RJ45 at least 1Gbps, so far the Netgate 1100 can cover this work for me.