Can I set a default IP address that connects to it for my network as shown below in the attachments? cuz it changes every time, and I recognize this is the best IP address for ping and internet speed.
You can not “invent” IP address on LTE interface, it’s assigned to you by your MNO. If speed varies (and doesn’t reach your subscription minimum speed), then complain to your MNO.
BTW, speed in LTE networks vary a lot as other users use more or less data. Usually the most usage is between late afternoon and midnight (so that’s when it’s expected to see worse performance) while during small hours (between 2am and 5am) usage is usually lower and hence performance is the best. This normally doesn’t depend on particular WAN IP your LTE router might get.