The 5G toolbox is a licensed product. If it is included in your license, you can install it using the Add-Ons explorer.
The 5G toolbox is not listed as one of the products where a trial is available (see here). In that case, I would suggest reaching out to your account rep. If you don't know who that is, use this form: https://www.mathworks.com/company/aboutus/contact_us/contact_sales.html
