Port forwarding on Orbi not working

> [...] I have an at&t modem [...]

   Maker?  Model number?

> [...] (which is not acting as a router) [...]

   Says who?  (Hint: Disabling the wireless-network radios on a wireless
router does not stop it from being a router.)

> [...] connected to my Orbi. [...]

   Model number?  Firmware version?

> [...] the orbi is on router mode. [...]

   Ok.

> [...] I have done port forwarding on port 25565 on both udp/tcp on
> both the at&t modem and the orbi router. [...]

   If you're configuring port forwarding on your "the at&t modem", then
it's not a simple modem.  Port forwarding is a router function, not a
modem function.

   For the usual problems with port forwarding, see:

      https://community.netgear.com/t5/x/x/m-p/1859106

I'd guess "1", because your "modem" is actually a modem+router.