aircrack-ptw only works with .cap files containing traffic generated by using aireplay in attack mode 3 (replaying ARP requests). It can't be used with .cap files containing arbitrary traffic.

Unfortunately aireplay does not currently work on the N800, so without the possibility of injecting ARP packets using the N800 you can't generate a suitable .cap file, therefore you can't really use aircrack-ptw on the N800.

Of course you can test aircrack-ptw on the N800 by generating a suitable .cap file using a laptop running aireplay to do the ARP packet inkection. But if you are going to use a laptop you might as well run airodump-ng and aircrack-ng on the laptop as well.

If you want to do the whole thing on a standard N800 you are out of luck I am afraid