Do you have a device running macOS?
If so, with Obsidian quit on the phone, you could use the Finder in macOS to go to iCloud Drive/Obsidian/{VAULTNAME}/.obsidian/ (you may need to set the Finder to show hidden files and folders to see .obsidian).
In there, rename or move the .workspace-mobile.json file. Wait a bit for iCloud to sync to the phone. Try to open Obsidian.
If you were using the core Workspaces plugin on the phone, you may also have to move or rename the workspaces.json (with a s) as well.
If you are comfortable doing the above, I’d give that a try.