Issue solved. After a lot of reading I realised I made some wrong statements. The thruth is that:
- The router had a Sputnik install based on wrong firmware version - it was for v3 hardware, not v4
- Because of that router ignored all v4 specific upgrades.
I was able to succesfully flash DD-WRT mini generic version via Web interface and I have fully working, DD-WRT enabled router now.
And just for reference - I found a website with old firmwares! http://download.modem-help.co.uk/ has a huge archive, including files for my device.