Need firmware for RSH A400-S812

[zygd]

Manufacturer: RSH-TECH

http://www.rshtech.cn/product/53238292.html

Product: RSH A400-S812
Other product id: CVJI-E425
ro.product.model: RSH-A400-S812-V1.0
ro.product.device: n200_RSH6210a400
identifier I have found in compatible recovery.img: m8m2_n200_2G
Board: n200
WiFi: AP6210
CPU: S812
RAM: 2G
NAND: 8G.

RSH is ignoring my requests about firmware, they share only manual in pdf format. Orginal device supplier gave me not fully compatible firmware which made the device almost unusable, but not bricked so I can still burn new firmware into. I don't count for any stock version, i wasted too much time on searching for it, but I need anything which make the device usable again.
Can You suggest any good firmware for these specs?

EDIT: I have found that my device is compatible with CX-S806 device. It is also built on n200 and have similar specification. Firmwares (i tried 3 from different dates) fit.
There is problem with remote controller, but only needs fixes in remote.conf file. I have got this file from my original firmware so this is not an issue.
Some things on CX-S806 works better than on my factory firmware but there are other issues like not compatible apps in Google Play which were compatible on factory firmware. So i am going farther with my search.

[zygd]

The best I have found is latest Probox2 Ex (not plus) firmware. No more issue with SD card always mounted as read only. Now I can use SDFix to make a SD opened for all apps like it was before KitKat.
No more issues with non compatible apps in playstore, which where compatible before this firmware game started.
Only 360p on newest YT app, as usuall. It is bug in firmwares or in amlogic S8x cpus. 720p in older YT app is software decoding only.
The only problem is Probox2 Ex has originally S802-H inside. Kernel driver is compatible beetween S802 and S812 but I am not sure if this way all S812 specific features are used.
Watched Antutu Video Tester samples and I am not so sure if my factory firmware was better or the same. There are some clips playing quite bad.