First of all, I know little to nothing about the merits of different brands/models of
terminal adapters (TA's) because the area where I live is too "fringe" to support fully modern speeds (the ABSOLUTE MAX we can get here from
any standard infrastructure vendor is ~300Mb/s), so I will not speak to that other than yes, a device supporting DOCSIS 3.1 capability should be the minimum target for a "cable" Internet connection. But for routers, the Orbi looks like it should be fine, assuming it meets your needs (up to 75 wireless devices, that is).
As for routing/access point/meshing, I have been looking to upgrade my current setup for some time with some kind of newer system, but right now there really isn't anything that does what _I_ want (which is to keep my existing router BUT only upgrade my access points), so I don't have any current favorites. Most mesh systems come with a "main" router unit and some number of satellite units. The Internet you get from the wall does not determine what kind/brand/whatever mesh system you are allowed to use UNLESS your ISP uses
carrier-grade NAT and your system of choice has some weird issue with that, but this is not common. So long as your TA has at least one
RJ45 socket on it, you just run a cable from that socket to the "WAN" port on the base unit of your system and the rest is just configuration. The router system doesn't care where the Internet comes from, it just expects that it be connected to that WAN port. Everything connected to and upstream of the WAN port is considered "outside" of your personal network, and everything else is considered "inside" (LAN).
I am going to assume that you have some wired devices and some wireless devices. Wired devices should be easy, they just get plugged into any unused LAN port on whichever device is closest to their location. The cable you use should be designated at least as "CAT5e" (
not the "ordinary" CAT5), though if you are going to be purchasing new cables for any of your wired devices, I would just step up directly to CAT6. For simplicity's sake, you should probably not use cables longer than 100ft/30m. The base unit on the Orbi looks like it has three of these (LAN) ports, and the satellites each have two. Keep this limit in mind when deciding which of your devices will go where (or else get a small Ethernet switch to allow for additional ports where you need them). Wireless devices are a little more complicated because location and interference actually matter. The good news is that the Orbi supports WPA3, which is the newest WiFi encryption protocol, but none of that will matter if your devices are older and only support up to WPA2 or WPA (or even WEP but really you should be getting rid of devices that only support WEP). If you are not sure then just choose the WPA+WPA2 option.
Placement of your base and satellites is going to be very important. Assuming you don't have access to any kind of stumbler software or other device to measure WiFi strength, then you will just need to "best guess" your placement. When doing so, you will need to take the physical design of the building into account. The base unit must be connected via a physical Ethernet cable to your TA, and the satellites get placed somewhere else to repeat the signal to more remote locations. Remember that the wireless signal broadcast by each unit is going to be
toroidal or hemispherical (dome) in shape, and that it decays with distance, so when deciding where to put the satellites, it might be tempting to put them directly in the distant area(s) where you will most commonly be using the signal (CS----------B----------SC), but that is not the best method since the distance between the
Base and
Satellite means the signal coming into the satellites is weakened and can't be repeated at full speed for the
Client(s). Instead, it is better to arrange them nearer to the base (C-----S-----B-----S-----C) because a satellite (and therefore all the clients connected to it) can't talk to the main base unit any faster than the speed the satellite can "hear" from the base at that satellite's current location. Additionally there is nothing saying the base unit "must" be physically located right next to your main TA (other than the length of the cable), so don't be afraid to run a loooong cable from your TA to wherever the base unit is going to live if it means better coverage overall. Remember also that the WiFi coverage "bubble" broadcast by each unit is in three dimensions, so if coverage for basement/attic/upstairs/downstairs matters, your placement will also have to take that
Z-axis into account.
tl:dr; I don't see any reason NOT to use the Orbi for your Internet (especially if it's "free!"), assuming you follow some fairly obvious rules about placement and coverage.
