When I was to start with the second one, I realized that its boot loader (U-Boot) version was different and lacked of option 4 (Entr boot command line interface). It doesn't show it nor respond by pressing 4.
I needed to do some test at boot command line, so after doing some prudently research I took courage and rewrote mtd0 partition with mtd0 of first router (same router model and SoC - identical PCB at first look), using originals firmware's Flashw tool. I preserved intact the other partitions Config and Factory. Instantaneously after restart all lights gone off except the green internet light with very low intensity, and it didn't work again.
Previously I have backed up all partitions of original firmware including boot loader.
Is it possibly to use JTAG with this unit to rewrite original boot loader? Does somebody managed to use open source software JTAG tools and homemade parallel port JTAG cables with Ralink SoC?