Indeed, "Metropolitan Area Network" is what the acronym stands for, but that doesn't really answer the OP's question.
In the context of the ASUS router; as far as I can tell the "MAN interface" is physically the same interface as the WAN, except it has been configured for PPTP or L2TP. Which could make sense because in such a circumstance you might be connecting to a large corporate or academic network (MAN) rather that the internet (WAN).
Sorry I don't know, it's not something I've ever used.
But if I had to guess ...
What do you have set at WAN > Internet Connection > WAN Connection Type ?
I think the routes only apply if you have set PPTP or L2TP as the WAN connection type. I don't think it applies if you are using the VPN client (VPN > VPN Client).
But this is all speculation on my part.
I'll give it a shot today/tomorrow. And will report back. The way it sounds to me it's just a less elegant way to set which subnet should be routed through WAN and which through the MAN (PPTP/L2TP), compared with the OpenVPN interface, where it's laid out so clearly. Will try.
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