Get a couple short Ethernet cable from staples or amazon and connect it from the ont to the laptop direct and measure the speed. Then connect the ONT to the router and lan to your laptop. This will tell you if it’s the cable. Another option is a cable meter. You can get them on Amazon for under $20. https://www.amazon.com/ATian-Telephone-Tracker-Ethernet-Detector/dp/B00EDZ0C4C/ref=sr_1_16?dchild=1&keywords=Ethernet+meter&qid=1602727092&sr=8-16