Hi Guys,
I have found a very easy way to get back to SIM2.1 with my sons DM800se.

Leave JapharSIM inside.
Put DM800se into Browser Flash mode.
Flash the desired image which has the SSL84D inside with the Browser and Kill Box at rear.
DO not re-boot.!!
Open Box replace the JapharSIM with the old SIM2.1 and switch Power back ON.
Ready.

I did NOT wont to start messing around with Drivers and Kernel, so decide I will go this way.
Just waiting for a new 1TB Portable 2.5 slim SATA III HDD to install.
Cheers
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Please STAY SAFE and PROTECTED from COVID-19 wink
AJP