Practice nintex at home

The most straightforward way would be to talk to the IT dept. at your work to see if there is any way for them to set you up with a vpn that would allow you to access the network securely (along with any other security measures they may want you to take). In this way you could, for instance, use a work laptop at home connected to your wifi, but still able to make a connection to the intranet of your workplace (giving you access to sharepoint).

The only other option I could think of would be to sign up for a free Microsoft 365 Dev Account, which gives you access to a SharePoint instance: https://developer.microsoft.com/en-us/microsoft-365/dev-program

And then grab the trial of Nintex Workflow / Forms for Office 365: https://www.nintex.com/trial/#office365

Unfortunately there is no other official way that I know of to simply do dev / learning work for Nintex.

Hope this helps! Let me know if you have any other questions!