Nintex has a feature called "lazy approval" which allows emails to contain the approve links right from email.
Here is some information:
https://help.nintex.com/en-us/nintex2010/help/workflow/RootCategory/SharePoint/Nintex.Workflow.LazyA...
That is correct, anyone in the Company will be able to approve through email even if they are not connected to the Company's network.
We are also on Sharepoint on-prem and this was one of our requirements for our Executives to be able to approve/rejects from anywhere using email on their cellphones.