Nighthawk ac1750 won’t connect to internet

> Just installed arris T25 [...]

   Ok.  That's a cable-TV modem.  "installed" _how_, exactly?

> [...] netgear will not connect to the internet when plug the same
> cable that was connected to my computer, [...]

   Huh?  What, exactly, are you connecting to what, exactly?  (Hint: If
a device has different types of Ethernet ports, then "connected to
device" is not enough detail.)

> [...] tired of unplugging the modem [...]

   Which one?  From what?

   What, exactly, are you trying to do with these devices?

> Model: C6300|AC1750 Cable Modem Router Docsis 3.0

   That's a cable-TV modem+router.  It can't be used as a router with
some external modem (because it lacks a WAN/Internet Ethernet port to
which to connect an external modem).