You could try downloading the firmware and manually copying it to your Touch. See the
Kobo Firmware Releases wiki for information.
If you are correct in having a N905 (either N905 or N905B), the last supported firmware release was 4.5.9587. The Mark 3 firmware can be downloaded from Kobo at
Kobo Mark 3 firmware 4.5.9587. But double check your device -- the N905C (Touch Low Cost) uses the Mark 4 firmware.
To use the firmware, unpack the downloaded .zip file and copy the contents (the upgrade directory and the KoboRoot.tgz and manifest.md5sum files) into the .kobo directory on your Touch.
If this doesn't work, it is possible that your internal storage is having issues and may need to be replaced. See the
Kobo Touch : Need SD card image thread in the Developer's Forum for more help.
Good luck!