Reflashing a Chinese Seriallink SLK-e940 4G LTE Gateway Router with OpenWRT

Recently, I needed an LTE backup at a secondary home location with an unreliable primary internet connection. I was somewhat bewildered by the plethora of options available. I saw a weatherized Seriallink SLK-e940 on eBay for a very nice price, so I looked up the details on it. I found (from the help screenshots) that it appeared to run a modified version of OpenWRT. The image the seller provided showed inside the unit a mini-PCIe Quectel EC25-AF LTE modem, which I knew had good Linux support.