AX6600 (rax70) loses wired connection to some devices, wifi good.

> [...] I picked up a Netgear docsis 3.1 modem and the Nighthawn AX6600
> router. [...]

   Actual model numbers?  CMxxxx?  RAX70?  Look for "Model" on the
product labels.  Firmware versions?  What, exactly, is connected to
what, exactly?  (Hint: If a device has different types of ports, then
"connected to device" is not enough detail.)

> [...] 2 other computers (one on an ethernet splitter with laptop) and
> an xbox [...]

   What, exactly, does "an ethernet splitter" mean to you?

> [...] would not connect. [...]

   "not connect" 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.

   A clear description of what _does_ happen would be more useful
than a vague description of what does _not_ happen.

> I ran an ethernet cable directly from the modem to my computer, [...]

   Was that the only device which was connected to your (unspecified)
"the modem"?  How many devices are you connecting to your (unspecified)
"the modem" when you have problems?

   Possibly relevant:

      https://community.netgear.com/t5/x/x/m-p/1770096

> [...] Pretty sad when a $300 router has issues like this.

   You might want to wait a bit before blaming the router.