Hello, there is a youhua wr1200js router, I found an interesting article on how to increase the RAM to 512MB. maybe someone has already implemented from users?
Even if you have the tools and skills to even do that kind of project I doubt it makes any sense. (time, parts, risk...?)
Those wr1200js routers come standard with 128MB. Are you actually running out of memory for your use case? Doesn't it make more sense to buy a (secondhand) replacement with more memory. Other Chinese brands come with 256 or even 512MB (maximum for the MT7621) pre-installed and they can be found for relatively little money.