Access point mode but manually? (Cause I need DHCP on second router)

> [...] if use Access Point mode, I cannot assign different DNS servers
> to the computers on the second floor, which I really want.

   Why?

> Is there a way to manually use Access Point mode [...]

   "manually"?  A wireless access point is a wireless access point, and
it doesn't act as a DHCP server; the main router typically provides the
DHCP server.

> Is there a way to enable DHCP server in Access Point mode?

   Not in any useful way, so far as I know.

> Because if I use a static IP on the R7000, [...]

   What does that mean?  In router mode, an R7000 has two IP addresses,
one for each interface: LAN and WAN/Internet.

> [...] I can access computers on the R6400 but not vice versa.

   "access" from where?

   "can not" is not a useful problem description.  It does not say what
you did.  It does not say what happened when you did it.  As usual,
showing actual actions (commands) with their actual results (error
messages, LED indicators, ...) can be more helpful than vague
descriptions or interpretations.

   You might get better advice if you described the actual problem which
you are trying to solve, rather than asking how to implement some
particular "solution" ("different DNS servers") which may have nothing
to do with your actual problem.

   If you want to cascade two routers, which might let you run two DHCP
servers (by establishing two LAN subnets), and you don't mind the
potential problems from the resulting "double NAT", then you ought to be
able to make it work (within limits) by adding a static route to the outer

router.  See, for example:

      https://community.netgear.com/t5/x/x/m-p/1558939