I have finally received the router and I have realized that it's a single chip one, based on Qualcomm MDM9240. This device runs a full Linux and integrate wifi and a LTE modem in a single chipset, implemented in a LTE M2M module.
It's very similar to the router described in this post: Hacking on a MDM9207 based device
Here some pictures of the inside of the Dlink DWR-960 rev B.
In conclusion, there are many unsupported hardware blocks in this single-chip routers. They run an highly customized Linux and it's not obvious to replace their firmware.
I don't see any straightforward method for implementing OpenWRT on a similar platform.