I have the Hydra Pro 6 MR5500 from Linksys, but they have disable the vLan options completely in the webGUI json files. I found out using a spider on the webGUI address.
Now I have downloaded the stock firmware and unpacked it for now and have the Squashfs root. It does run on OpenWrt already.
In the firmware_rdp.ini file I have found out that it uses the following hardware/ firmware:
IPQ5018
qcn9000
But IPQ5018 support for OpenWrt seems to be a difficult issue?
So how can I use the information inside/ firmware packages and config to build a compatible OpenWrt image?
I see, reading the bootlog of the RE700X it differs from the MR5500 on platform base.
The RE700X bootlog shows the Maple V2 platform while the MR5500 uses the Maple V3 platform and according to Qualcomm the support for the Maple V3 states OpenWrt V15.0 compatible.
if you want to create a firmware based on an 8 yo release, not compatible with any official openwrt release, go for it, but please ask Qualcomm/Linksys about any issues you might run into, during and post compile.
There is some ongoing development happening for ipq60xx, but it'll take time (months++) - SOC development, support for the specific device may-or-may-not come afterwards. In general, never bet on future device support, always go with something that already is supported right now (at least in snapshots, if you feel frisky, maybe something that has an active pull request and people actively working on it).