There really ought/needs to be an on-screen message alerting users not to perform a Factory Reset on these older devices.
I do believe that those that were only ungradeable to OS 3.1 (like the 2100) cannot be salvaged after a Factory Restart. I think you need a device capable of at least OS 9.1 to be able to access the Roku servers and install the latest released OS available for the device.
Maybe someone knows of an alternate way, but normally @RokuDanny-R offers to help such users in your situation.
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee. Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102