So the only way to run an app on OpenWRT is by using a Docker container which requires a router with more ROM or a router with USB expansion, right? There is no other ways?
You could set up a network file system to store large files remotely and load them into the router RAM when needed, but then you'll need another server with large storage to hold and serve the files.
If the page is correct you have... ...not a lot of ram. 256Mb
Let that sink in. 256Mb of ram (what the router actually runs on) and 128Mb flash (what it boots from) . That is like 1999 desktop levels.
Don't feel bad, it is a "reasonable" amount for a present day router, but I'm going to put that in quotes.
Now, how big is the package you are trying to load, and what does it say it needs to run all alone?