"No updates available" message usually means you've either used the incorrect HWID in the RGN file's naming or maybe used the correct naming for the firmware it's made from but that naming doesn't align with the actual HWID of the device.

I think i mentioned earlier that the nuvi 25x9 series has 2 different hardware versions using 2 HWIDs, 1758 and 1947 the latter being the one with "C4" chip. They use different firmware and are not cross-flashable.

Best we confirm you have these things right before you do anything else, so stop any further flashing attempts until the HWID is confirmed. Using the wrong fw named the same as the device's HWID will brick it totally (potentially permanently hard-brick vs present temporary soft-brick). I want you to use a program called G7ToWin which works in preboot to show the device's actual HWID, then report back before proceeding. The full instructions and download links for both 32 and 64 bit versions are [Only registered and activated users can see links. ].