The popular mobile modems available on the market are Category 4. Please refer to the image below for details. In that case, you should connect the SIM card to SIM2 slot. If your operating system is installed on an SD card or USB thumb drive, you can also use the first mPCIe slot that is usually occupied by the mSATA SSD drive. If you are also using PC Engines APU3 or APU4, please make sure to connect the SIM card to SIM1 slot, and your modem to the middle mPCIe slot. This hardware combination works very well in our experience. In this tutorial, we are using Quectel EC25-E LTE modem with APU4D4 motherboard. Some modems aren't compatible with some motherboards. There are many 3G, 4G and LTE modems on the market, but not all of them work well with OpenWRT.
The first thing you need to do it to have compatible hardware. If that's not the case, please start by installing OpenWRT on your router. Please note this article assumes that you have OpenWRT already installed. This step by step tutorial will show you how to configure a 4G mPCIe cellular modem, and use it as the main internet connection in OpenWRT.
