MR2100 - Need to Manually set MTU (due to PMTUD failure)

I've been trying out the 3 big LTE networks in the UK for comparison (EE, Three, Vodafone), I'm sticking with Vodafone which has the best reception in my area, and this is where the MTU problem starts.

The MR2100 appears to do some kind of PMTUD for the LTE connection (Path MTU Discovery) and then relays this for new wifi connections to ensure connected devices fragment packets at the appropriate size for the network, which is great, and seemingly necessary as each LTE network appear to have fairly arbitrary and low MTUs.

It correctly determines the MTU for EE and Three, e.g it uses 1380 bytes for Three which is correct, however It uses 1500 bytes for Vodafone which is incorrect. Now I'm pretty sure I know why this is - because the initial connection on Vodafone _is_ in fact 1500, the problem is with Vodafone the effective MTU _always_ lowers over time, eventually settling on 1464 bytes - I can only speculate as to why, perhaps cell aggregation causes it to settle on a different path - Anyway, the MR2100 does not detect this, it only appears to attempt PMTUD upon the initial connection, and so of course all the connected devices will continue attempting to send 1500 for large enough packet... and you can probably guess what happens, things get flakey, website loading gets a bit annoying, HTTP requests that had a particularly big header will just vanish... uploading becomes almost unworkable.

It's possible to manually work around this by first using "PMTU black hole discovery", i.e bisect the MTU size by manually pinging different sizes starting with `ping -M do -s 1472 1.1.1.1` (-28 byte header), this is how I found the correct Vodafone MTU. Now I can set this for every - single - device... which works, but is incredibly annoying.

So my question is: How can I manually set the MTU on the MR2100 to centralise this, this used to be an ordinary setting for the modem on old routers, but there doesn't appear to be any setting for this in the Mr2100 user interface (192.168.1.1). I'm happy to fish around i the telnet console but don't know where to start or if it's even possible.