When Wifi is OFF on RUT950 you won’t be able to reach 192.168.2.254
When Wifi is enabled, configure RUT950 Hotspot → Radius server → Enable and Remote access enable. In Hotspot general settings select Authentication mode: Internal Radius.
Then on Ubiquity AP you’ll need static route to reach 192.168.2.254/24 subnet via 192.168.0.1.
After this you’ll be able to reach 192.168.2.254 IP, but how exactly it will behave needs some testing as I don’t have Ubiquity router on my hands.
RUT950 Hotspot is designed for RUT950 AP and not as external radius server for other manufacturer AP’s so if provided solution won’t work, I’d suggest for you to look into cloud based radius/captive portal providers such as: