MB Connect Line mbNET MDH810 Manual page 129

In addition, routers with a GSM/UMTS module and double SIM slot can switch between SIM1 and SIM2.
First, we need to specify a primary SIM card, which will always be verified or used by default. The
secondary SIM card is always the non-primary one.
Switching is based on two (selectable) criteria:

The SIM card fails to initialize, or to register on the mobile broadband network

Roaming is detected on the SIM
Page 129 of 226
Version: 3.3.5 – DR05 – 23.03.2017