DSL-AC51 firmware

Do you have an opinion for the DSL-AC51 model firmware? or compatible another model firmware ? This model is widely used in my country.

Check here: https://openwrt.org/toh/start?dataflt[Model*~]=ac51

There is a regular router (no DSL modem) RT-AC51 which is supported. If your box has an integral DSL modem, it's going to be entirely different inside. There doesn't seem to be any information about a "DSL-AC51" on sites like wikidevi.

DSL-AC51 and RT-AC51 with different hardware.

DSL-AC51 Mediatek MT7511T CPU

But RT-AC51 Mediatek MT7620A CPU

MT7511T is not Ralink-Mediatek, but from the TrendChip / EcoNet family of SoC's that now also belong to Mediatek.

These are MIPS 34Kc devices, and there are lots of Mediatek xDSL routers based on this SoC (also older ones with TC3182, RT63365, RT65168), however there seems to be no matching target in OpenWRT yet.

I wonder how difficult porting to this platform would be, unfortunately I currently don't have the skills and time.

You can try unpacking the firmware with https://github.com/vasvir/tcrevenge or try to get a serial console and poke around with br2684ctl etc.: