Is there a way to forward online calls to cell network?
Unless you have a particular reason for wanting to route through a secondary device, a more direct approach would be to investigate apps that are available to you that use wifi-calling. These can be used even in airplane mode (with wifi subsequently turned back on) and behave the same way whether you are in your home country or abroad - ie no roaming charges.
These are often carrier specific (at least in my experience).