Thanks to ZTE Germany you can now download the Nougat B08 Firmware as SD Card Update for the A2017G from their homepage (for example if you have problems with fota).
From the original message from ZTE Germany:
Please note that the the preferred method to Update is from the internal SD and not from an external SD card.
I would also recommend to remove the SD card before updating because there have been reports (with older versions), that firmware updates can corrupt the external SD card. I personally never had problems with that but if you have sensitive data on your microsd you should consider removing it before updating.