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!