internet keeps dropping

> [...] new router in accesspoint mode behind a cable modem. [...]

   Your "new router" is an RBR20?  How, exactly, did you get it into
"accesspoint mode"?

   What is your (unspecified) "a cable modem"?  Is it really a modem, or
is it a modem+router?  Which way is "behind"?  What, exactly, do you
have, and what, exactly, is connected to what, exactly?  (Hint: If a
device has different types of Ethernet ports, then "connected to device"
is not enough detail.)

> [...] What am I doing wrong?

   What, exactly, are you doing, with what, exactly?