AC1200 wifi cable modem router IP address being used by multiple devices

> We often cannot connect to the internet. [...]

   "cannot connect" _what_ "to the internet"?

> [...] The error message states: "Another device on the network is
> using your computer's IP address. Try connecting again later.If you
> continue to have problems, change the IP address of the other device."

   That's a connect-to-the-LAN problem, not a connect-to-the-Internet
problem.

   That usually indicates a problem with way the devices on your LAN are
configured, not a problem with your router.

> [...] We upgraded our internet service with Xfinity from 200 mb to
> 800mb within the last week and we continue to have this same problem.
> [...]

   Not amazing to me.  If you didn't change anything which might cause
that problem, then I wouldn't expect it to heal itself.

   (Note: "m" = "milli" = 1/1000. "M" = "mega" = 1000000.)

> [...] I called them and they said that I had to contac the
> manufacturer of the modem as only they have access to renew the IP
> address. [...]

   Someone lost the part of that message which might have made sense.

   You router has two IP addresses: One for its WAN/Internet interface,
and one for its LAN interface.

   Its WAN/Internet interface is used to communicate with your ISP.  You
have no control over the address of that interface, and it's not part of
this problem.

   Its LAN interface is used to communicate with your client devices
(wired and wireless). It's address is normally "192.168.0.1", and it's
also not part of this problem.

   Your client devices should have IP addresses like "192.168.0.x",
where "x" is between 2 and 254 (inclusive), and "x" is different for
every device.

   When a device starts up, it tries to configure its network interface
with some address.  Normally it would use DHCP to ask (the DHCP server
in) the router which address it should use.  When it tries to configure
the interface, it first checks to see if some other device is already
using the proposed address.  If it finds that that address is already
being used, then you get a message like the one you quoted.

   One mystery is _which_ "Another device" is conflicting.  The other
mystery is which device is not using DHCP to get its address.

   Generally, the router's DHCP server will not grant the same address
to multiple devices.  So, if there's an address conflict, it's normally
because someone has configured one of your devices with a static address
-- one which was explicitly specified by you/someone, rather than one
which was obtained from the router's DHCP server.

   I know nothing about your devices, or who configured which how, but
you might try disabling some of them to try to determine which one is
causing the problem.  (Your (unspecified) computer could be the culprit,
too.)

> [...] Is it that we have too many devices working and the modem can't
> handle them all? [...]

   Unlikely. How many devices have you?

> [...] Do I need to upgrade my modem? [...]

   I doubt that that would help.