In most cases - one shouldn't have to worry about MTU sizing...
PMTU, which is end to end, usually handles things...
en.wikipedia.org
There are cases - for example, client VPN apps like GlobalProtect and AnyConnect that do this badly...
MTS/MSS sizing - this was important for
Trumpet Winsock back in the Windows 3.1 days, as that was a bit of a science project...
With IPv4 PMTUD rarely works, it's really frustrating. Bad enough on a private network, almost useless on the internet. My company's network has been specifically designed to allow 1664 byte frames end to end (even the routers can support it, which is a somewhat recent and welcome update from Cisco and Juniper) so we can support fully encrypted 1500 byte packets, but on either end customers have 1500 or less and firewalls that block ICMP, resulting in tons of fragmentation and performance that is 10% of what it should be. Thus we end up having to MSS clamp/intercept on pretty much every encrypted link.
If we ever do see IPv6 and people don't mess with it by blocking the critical ICMP packets, then it should finally work as intended. PMTUD is considered a critical aspect in the IPv6 world. Then again so are persistent subnet assignments and lots of other things that the ISPs have managed to muck up.
In this case, as long as your Asus is set to the lowest in the path, then PMTUD does (in a way) work, as your LAN machines can easily detect the MTU of the LAN interface or even the WAN, and should adjust their MSS accordingly to fit it, even if the remote endpoint does not respond. Since the pipe beyond that is larger, there shouldn't be any chance of fragmenting, at least with non-encrypted stuff. But from what I've seen here, once you start using VPNs on these routers, they seem to default to 1500 (and set their MSS accordingly within that) and are either not doing PMTUD, or for whatever reason not getting the response from the router WAN. Maybe some are better than others, just seems like an issue for many here.
If your ISP supports 1500 then you're basically golden, unless the VPN software was written by morons it should work without having to fragment anything. But those with PPPoE, WISP, satellite, or even several fiber providers (for unknown to me reasons, since FIOS manages to do 1500 just fine), then it is a good idea to get your WAN MTU set exactly right, and if you're using VPN, at least check and see if it is detecting MTU properly and not fragmenting anything.
Of course it is all relative too, someone using a VPN for average daily internet use may not even notice heavy fragmentation, and it isn't an issue. Those who are trying to get as much speed as they can out of VPN on these routers on the other hand can benefit a lot from making sure it is tuned just right.
Then there's the fact that if the remote end is less than 1500 (or less than yours) and PMTUD isn't functioning, then there's nothing you can do, you're going to fragment regardless, but that is pretty rare, most stuff you're connecting to is going to be off commercial internet connectivity which is almost always going to be 1500.
