e-SIM support is haphazard. Most MVNOs do not support e-SIM and even the big four that do, it is not widely supported. This makes a complete mess and makes e-SIM with a bumpy start to standards.
With FSP Wireless, you would simply download the FSP Wireless App from the Apple App Store, or the Google Play Store. From there, you would log into your account, and select use e-SIM. Follow the instructions, and in moments – your account will be tied to your e-SIM.