What issues are you seeing?
I have multiple setups on version from 12 to 14 that have multiple subnets and haven't ever ran into a issue with it or having to modify anything to make it work.
By default 3CX will see "10.0.0.0/8,169.254.0.0/16,172.16.0.0/12,192.168.0.0/16" as the local subnets so...