R7800 set in Access Point Mode not working for Wireless

> The Netgear router is directly connected with the router from my
> provider Yellow Ethernet port of the Netgear router. [...]

   Does that make any sense to you when _you_ read it?

   Do you mean that you connected the "Internet" (yellow) Port of the
R7800 to some LAN Ethernet port on your (unspecified) "the router from
my provider", or what?

> [...] What kind of extra info do you need more?

   What is this "another router (from my provider)"?  Has it a maker and
model number?

> [...] The reason why I'm using LAN Subnet 192.168.2.1 is because of
> previous router configurations [...]

   Ok.  Does this "another router (from my provider)" have anything like
an "Attached Devices" or "DHCP Clients" report, and does the
R7800-as-wireless-access-point appear in it?

> [...] The configuration of the R7800 is just basic. [...]

   I would expect a "just basic" configuration of an R7800 to give you a
router, not a wireless access point.  Can you get a web browser to talk
to the management web site of the R7800?  Does it say anything which
suggests that it's acting as a WAP?

> [...] I did follow the procedures on my screen during the
> configuration. [...]

   I wasn't there, and my psychic powers are very weak.  As usual,
showing actual actions with their actual results (error messages, LED
indicators, ...) can be more helpful than vague descriptions or
interpretations.

> As soon as I want to open a webpage I get instantly the message "The
> Connection Is Not Secure". No matter which webpage I want to visit.

   "No matter which" tells me nothing about what you tried.  (Or with
which web browser, on which computer/device/operating-system.)  I've
already offered an explanation of the usual cause for such messages.

   Is this behavior any different if you connect the computer/device
directly to the main router?  Or did you mean "No matter which webpage"
on the management web site of the R7800?  Or what?

> If I disconnect the network cable coming from the provider-router and
> connecting into one of the other Ethernet ports of the Netgear router
> (port 2, 3 or 4), the wireless connection to the internet is working
> but I don't have an internet connection anymore with the wired
> connection from my computer connected to the Netgear router (port 1).

   Assuming that your wireless device is connected to the R7800, and not
to the main router, then I don't have a good explanation for that.

   It might be interesting to see the IP addresses of all these devices.

> [...] I don't have an internet connection [...]

   What are the actual symptoms of that?  Is it a DNS problem, or an
actual connectivity problem, or what?  For example, does a command like,
say:

      ping 8.8.8.8

   work better than:

      ping dns.google

?

   For a good time, or if you get desperate, you might try disconnecting
the R7800, resetting its settings, and configuring it afresh.

   Visit http://netgear.com/support , put in your model number, and look
for Documentation.  Get the User Manual.  Read.  Look for "Use the
Router as a WiFi Access Point".

   Connect one computer (only) to the R7800, and as soon as you can get
past any wizard, follow those instructions to do a manual configuration
of the R7800-as-WAP.