Pool contractor needs a 2.4 GH signal.........

> any dual band router has a 2.4ghz signal.

   The problem is not that the 2.4GHz signal does not exist.  The
potential problem is that the 5GHz signal also exists, and on an Orbi,
with a single SSID, the contractor's mobile device can't be connected
reliably/exclusively using the 2.4GHz radio.

   Many Internet-of-Junk devices can use only the 2.4GHz radio band.  In
itself, that's not a problem.

   For reasons which I don't thoroughly understand, many mobile apps
which are used to configure (and manage) these IoJ devices require that
the mobile device also be using the 2.4GHz band, at least during the
configuration process.  With the simplified (that is,
limited-flexibility) wireless controls of Orbi routers, arranging that
can be difficult (if not impossible?).

   After the IoJ gizmo has been configured, the presence of a 5GHz
signal is not a problem, because the IoJ gizmo itself won't notice it.
The problem occurs during the configuration of the IoJ gizmo, when the
mobile device which is used to do that configuration can't be forced to
use the 2.4GHz radio band.

   On older/non-Orbi routers, it's generally possible to specify a
unique SSID for each radio, or to disable the 5GHz radio(s) temporarily.
Either way, the mobile device can be forced to use the 2.4GHz radio
band during that configuration procedure.  I have not seen a reasonable
solution to this problem for an Orbi, which seems to allow only one SSID
for all radios, and does not seem to offer an option to disable
particular radios.

   And a typical mobile device does not allow its user to choose the
radio band, only the SSID.

   Configuring a "guest network" on an Orbi router won't help, unless
there's a way to make it a 2.4GHz-only "guest network", which, in my
quick glance at the User Manual, I didn't see a way to do.

> [...] with 2 or 3 extenders in my house. [...]

   If the geography is such that it makes sense for the pool gizmo to
connect to one of those (unspecified) "extenders", and if that extender
allows its 5GHz radio to be disabled or renamed (temporarily), then
using it might be a way around this (Orbi-related) problem.

   I dimly recall reading a report here where some victim walked away
from his Orbi until its 5GHz signal got weak enough that his mobile
device selected the (stronger) 2.4GHz on its own.  Not a reliable
scheme, I thought, but it did have some potential for the desperate.

> [...] it shouldnt need the guest account.

   The value of a "guest network" is its ability to be fiddled with
without disturbing the existing (non-"guest") configuration.  But, if
you can't do the required fiddling (band-specific SSID, disable specific
radios) on an Orbi, then it wouldn't help.

> I don't think your technician is much of a networking guy either.

   Perhaps, but I'd guess that he has far more (unhappy) experiences
with this specific IoJ gizmo (and its app) than anyone here.