I believe the OP has intentionally turned DHCP off in their router. I went ahead and setup a test network without DHCP and I found the issue the OP is reporting. It seems that neither the saving of the wireless connection (which essentially fails due to I assume incomplete connection details) nor connecting then specifying details manually after the fact stick. Setting the manual connection offline in My OSMC gets reset when you connect to the ssid.
I found that if you turn the DHCP server on, connect to this network, then manually change your IP, this works fine and you can turn your DHCP server off at that point and it survives a reboot and everything is tickety boo.
I don’t know if you want to call this a bug or a limitation. Either way I suppose the solution for now is to either turn DHCP back on at least for the initial setting or to manually establish the link with connman.