Modifying a Symbian^1 (S60v5) device like the Nokia 5800 requires specific file formats to overwrite the factory operating system.
“Force Flash Detected.” “Ignored Dead Phone Check.”
Dedicated retro-mobile communities and specialized subreddits maintain Google Drive or Mega mirrors containing classic "hot" ROM releases.
hstsethi/awesome-symbian: An Awesome List about ... - GitHub
This is the most critical part of the keyword. Unlike modern Android phones that use .zip or .img files, Nokia S60v5 phones used .rpkg files inside a .rofs2 (Read-Only File System) package. An RPKG is essentially a firmware resource file used by Nokia’s flashing tools (like JAF, Phoenix, or Vanilla).