GPD posted this in their IGG. Too bad it's lacking important details like why would this antenna be any better than the one they installed already. Shouldn't they have already used antennas that match the frequencies of the modem?
WIN Max 2 4G Tips
If your 4G can connect but it doesn't have internet, you can try to buy and replace this antenna. Different area’s receive signal strong or weak depends on the antenna.
They at least know there are performance issues with their modem but I'm not sure they actually know why. Just like they're not sure if the Win Max 2 SD card issues are FW or HW problems.
Plus some people actually do get speeds within the realm of the very old LTE Category 4 standard the modem's using. I at least saw one person getting 20 - 30 Mbps download while their modern 4G phone gets 100+ Mbps on Verizon (ie. same as your carrier).