I think you skipped the part where someone needs to make a openwrt firmware image that actually supports the E4R. I see there is a thread started recently about adding support, but it doesn't appear much progress has been made.
The exploit firmware you listed are made solely to bypass the firmware signature requirements (without soldering a serial port or attaching directly to the flash chip), but it can't succeed without already having an actual working firmware image.
While you could theoretically test the deco_all_webfailsafe_faux_fw_tftp.bin exploit itself by following the exploit installation procedure up to step 6 and verifying that tftp server sees a request for an initramfs image, without a working initramfs for your device you won't see any further success.