Theoretically: Mendix might change the IP-address without warning or justification since it remains a free node.
Practically: yes, the IP-address will not change.
So go ahead and trust the IP-address, but to be safe, also add a automated regular check that will alert you when the IP-address has changed. Triggering ‘ ping appname-sandbox.mxapps.io‘ and comparing the result to yesterdays result will do.