Although I cannot answer your specific question about the serial port issues...
This device doesn't appear to be supported by OpenWrt.
This would mean that there are probably very few people here who will have used this device. It does appear that some work was done to try to get official OpenWrt running on the unit, but it seems that it stalled out.
Therefore, it may make sense to try other forums where the device is supported if you wish to gain access this way. Otherwise, even if you get the serial port working properly, you probably won't be able to do anything interesting since official OpenWrt isn't supported, anyway.