I found an inexpensive noname-router with specifications similar to the A5-V11. As far as I understand, this is some kind of clone, and it was used for many different variations in different countries, but externally and internally they are all almost the same. CPU RT5350F, RAM 32Mb (M12L2561616A), ROM 4Mb (GD25Q32). Firmware dump with A5-V11 does not give anything of course. The firmware is only U-Boot too, and I don’t know how to enter it (holding the power button does not work). Who knows more: what is this router copied from, how to flash it, which U-Boot is suitable and how to enter it, where are the UART pins located? Answer, please!
4/32 MB is already a dead end for current OpenWrt (min 8/64 MB required).
I collected custom firmware for A5-V11, there is enough for my needs.
That device is long out of support for OpenWRT firmware...the last version was 17.0.7
The oldest version currently in support is 19.07.8.
As @tmomas mentioned, it doesn't meet the current standards for flash and RAM.
Additionally, anyone doing development would need to actually have possession of the device to build and test firmware for it.
You are basically on your own.
Start researching on TechInfoDepot.