I flashed my Elecom WRC-1750GHBK2-I with OpenWRT 19.07.4 by using its factory web interface. Going to Network -> Wireless, A 2.4GHz radio (radio0) exists, but the 5 Ghz radio is not on the page. I had been using both radios with the factory firmware before flashing. Factory firmware is the latest version.
I configured radio0, try to connect one device to it and can see it in the OpenWRT gui.
Router's WAN port is not connected.
I tried to reboot (System -> Reboot) and it seems that OpenWRT is reset back to freshly installed state: (1) it forgets the new root password I set and says "no password set!" and radio0 (2.4GHz) becomes disabled. The 5GHz radio is still missing.
Just to make sure, I worry bricking the router. The OpenWRT web interface runs on the router. Could I flash your linked image using that? ie System -> Backup/Flash Firmware -> Flash new firmware image (Upload a sysupgrade-compatible image here to replace the running firmware.)?
Thank you very much for helping. I appreciate your patience in linking me details.
The 5g radio is found and settings are saving. The router is working for me. I end of up using the sysupgrade image (19.07.4) below as it matches the version of initramfs-factory (19.07.4) that I first installed. Serge's link earlier is to 19.07.3. I flash the sysupgrade image via OpenWRT's GUI.