Did you save the settings after changing the Subnet configurations? To WW point above, given that the subnetting from parent subnet is automatically done, you might have to change address on the inside.
"The child subnet is automatically calculated from the parent subnet. This simplifies the need to manually calculate the size of each child subnet. Each MG will automatically assigned the first usable IP of that child subnet. This behavior is intended to not be configurable in order to make the MG assignments predictable."
As an aside, if you are tying to open a port for inbound traffic, you would need a public static IP on the SIM, if you don't have that, CGNAT won't know what to do with inbound traffic since flowstate is only created outbound from the UE.