Thank you very much for trying to emulate it. Appreciate the effort!So I had a good idea that does not seem to be valid.
TPLINK has emulators for all their routers so you can try out configurations. So the first thing I see is "C6" is not on the list...have no clue why. So I figure since they run fairly standard software I will try similar models. So when I set the IP static it rejects the 254. This technically is not valid for ethernet and I did not try a PPPoE configuration. So I tried 252 just for laughs. The stupid thing set it back to 255.255.255.0.
Maybe you can find something useful. otherwise we need to see if someone who has this device can try it for you to see if it really works this way or it is a bug in the emulator
It can be used on point to point links. Not sure if pppoe qualifies as point to point. It is invalid for ethernet BUT cisco commercial routers support it. It is commonly done by ISP for their connection between routers to conserve ips.ISP are human too, maybe they made a mistake. .254 for MASK as known is illegal. The smallest valid subnet is .252.