Is your SIM card inserted to the tablet correctly?
Android 7-inch Tablet PC w/ Dual SIM Slots, QuadCore 2GB RAM/16GB ROM expandable up to 512GB, WiFi Enabled, BT Enabled - Walmart.com
Amazon.com : PRITOM 10'' Tablet Android 10 Phone Tablet with SIM Slot, 32GB Quad Core, IPS Touchscreen, 8MP Rear Camera WiFi GPS Bluetooth USB C, Support 3G Phone Call, Black : Electronics