I got my Banana Pi R4 about 2 weeks ago and booted the pre-installed OpenWRT version. I tried updating OpenWRT using sysupgrade and when after a while nothing happened (it quit the SSH session and UART connection) I tried restarting the device by unplugging the power. Since then I have neither been able to get UART output (it worked before, so the adapter and configurations should be correct) nor a SSH connection.
When I power on the device, the red power LED is on and after 5 - 10s the blue LED goes on too (no matter what boot option I select). The board gets warm, so the CPU seems to be doing something.
I tried booting a new image from an SD card but got no output on the UART. The way I understood it there should always be an output, already in BootROM. I also tried the mtkuartboot tool and a connection via JTAG (however, I am not sure whether the pins are enabled). Nothing has worked. What is the most likely cause? Hardware issue? Are there additional debugging options like e.g. a jumper pin that enables some sort of recovery mode?