100 Mbps would be disappointing to me as well. Because of its rental fees alone, I wouldn't even bother tweaking the Panoramic. Return it.
The Orbi does NOT have a modem. It needs to connect to another device. Remember, you'll need either a DOCSIS modem or an Optical Network Terminal (ONT) to connect onto the Cox network. DOCSIS uses the cable-TV network (coaxial). If Cox has fiber optic cable in your neighborhood, they'll connect an ONT on or in your house. The ONT will then connect to your Orbi. If your friend doesn't have a DOCSIS, he has an ONT. The Orbi needs to connect to another device.
"...Mesh vs. Gateway for WiFi..." A mesh and a gateway are two different things. Every home network has a gateway router...but not all home networks have a mesh. Your gateway is the Access Point to/from your home network. If you want to shop online, you go through the gateway. If someone wants to send you a file, it goes through your gateway.
A mesh extends the wireless signals from your gateway. For example, the wireless signal will start at the Orbi gateway...an Orbi Satellite will receive it...and then the Orbi Satellite will relay the signal to your laptop. It's a "mesh" of wireless devices to rebroadcast the original signal from your gateway. Not all home networks have these rebroadcasting devices.
Before returning your Panoramic, we need to recommend both a DOCSIS and a router.
First, do you have Cox telephone service? If so, it'd be nice to combine both telephone and Internet into 1 device. However, if you want that 1 device, you'll have to buy it from Cox. For example, the ARRIS/Surfboard TM3402 DOCSIS supports both Gigablast & Cox Voice but you can't buy it retail. You can buy its DOCSIS equivalent, but it won't have the eMTA portion for telephone service.
If you do have Cox telephone service but don't care about combining, Cox, by FCC law, has to freely issue you a telephone modem. Freely. Don't lose it because someday you'll have to return it.
Second, after that choice, WiderMouthOpen is your best source for hardware.