Nighthawk x8 d8500

> [...] I have a hybrid fibre coaxial cable. [...]

   And some kind of cable modem?  Or what?

> [...] I have the netgear nighthawk x8 d8500 modem [...]

   The D8500 is a DSL modem+router.  How, exactly, are you connecting it
to what, exactly? (Hint: If a device has different types of Ethernet
ports, then "connected to device" is not enough detail.)

   Was the D8500 previously used with some kind of DSL service?  Have
you done a settings reset on it to give it a fresh start with this new
service environment?

> [...] I have read a lot of forums [...]

   Thanks for the helpful links.

   Think, for a moment, about how little useful information that
(non-)description conveys to the non-psychics in your audience.

> [...] I have tried everything [...]

   Then it's hopeless, and you should quit now.  Or, perhaps you
didn't actually try "everything", but see "Think, [...]", above.

> [...] When I plug in the TPLINK modem supplied by my ISP [...]

   "plug [it] in[to]" _what_?  Has your (unspecified) "the TPLINK modem
supplied by my ISP" a model number?  (See also, "Hint: [...]", above.)

> [...] I hope someone can help me step by step

   I'd bet you do.  Sadly, you might need to provide even a little basic
information first.  (Or try a psychic hot-line.)

   Visit http://netgear.com/support , put in your model number, and look
for Documentation.  Get the User Manual.  Look for "Set Up the Modem
Router for Cable Service"?