When you do "uci commit" and a "/etc/init.d network restart", then with "uci show network" the MTU setting will be shown. But the GUI doesnt know anything of!
In my tests, the MTU creates a lote of trouble, special case, with a webserive at port 80, just a oneway communication could be "established". For me, it seem like the RUTOS packet split isnt working properly. So, there semms to be a really narrwo band of situations, where it works properly.
Meanwhile, I removed all MTU changes everywhere.