I looked around but didn’t find a connman setting to change MTU in their documentation.
You can change it on the fly using sudo ifconfig wlan0 mtu 1200, where 1200 is obviously the MTU size you want. To confirm it worked, do an ifconfig wlan0 | grep -i mtu and look at the MTU number.
Doing the change this way though will not survive a reboot though, it’ll be reset back. If you wanted to make it permanent, execute the command sudo nano /etc/rc.local and add the following line BEFORE exit0:
ifconfig wlan0 mtu 1200
Save and exit using Ctrl+x, then y, then Enter.
NOTE: If you’re using a wired connection, change wlan0 to eth0.