Why does my Android device with a connect+ lose the configured APN?

Unless otherwise agreed, your connect+ SIM is a multi-IMSI card.

Android stores the APN settings per IMSI. An IMSI identifies a SIM card in the mobile network. To ensure the widest possible network coverage, several IMSIs are stored on your connect+ SIM card, which can change depending on the region and local network availability. It is therefore possible that your APN setting may be lost. Therefore, please make sure that you store the APN in your Android settings for every possible combination of MCC and MNC. You can find an overview in this help article.

Depending on the Android version, you can set the APN in the device settings under mobile phone settings by selecting "Add new APN".