flashing custom ROM with "Android.system recovery <3e>"?

Hello forum,

I have a HTC Desire 816 dual SIM and I would like to install Cyanogenmod.
Surprisingly, the recovery is not the HTC stock recovery, instead it is "Android.system recovery <3e>"
like this: http://ift.tt/2am2wOA

This recovery behaves quite different, so I can not follow the standard procedure to flash a custom rom on HTC 816g dual SIM.

What works:

• I was able to root the phone (using Kingo Root)
• I have a working adb root access to the device
• I can boot into fastboot
• Obviously Android.system recovery has access to external SD card as it can "backup user data" there

My Problems:

• I am not able to install a custom recovery
• I followed the instructions from http://ift.tt/2a9chfK and I tried all 3 ways mentioned at http://ift.tt/1ZaHm6g but the devise still keeps on booting into "Android.system recovery" (Vol. down. button).

My questions:

• Is it possible to flash a custom ROM using this "Android.system recovery"?
• How can i perform a backup of the stock ROM before flashing (this is essential)?
• How can I restore the stock ROM from the backup using this "Android.system recovery"?

Thanks for your support

Relaxy

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