An APN tells the phone how to establish its mobile-data connection. Some eSIMs configure it automatically; others need values supplied in their installation instructions.
Before changing it
Open the instructions for the exact eSIM in SimFuse. Do not copy an APN from a different plan, use a value from a forum or erase an existing value just because data is not working.
If no APN is supplied and the connection fails, ask support to confirm the required setting. An empty app field does not prove the phone's APN should be blank.
Where to look
On iPhone, an editable APN may appear under Settings > Cellular or Mobile Data > the line > Cellular Data Network. The menu can be hidden by the carrier configuration.
On Android, look in the selected SIM's mobile-network settings for Access Point Names. On Samsung this is usually under Connections > Mobile networks.
Enter only the supplied values, save and select the profile if required. Leave username, password and other fields as instructed. Do not invent hotspot APN values.
After the change, briefly toggle airplane mode or restart and test again. If settings are locked or absent, contact support rather than installing an unknown configuration profile.