A reflecting telescope with have an aperture - 114mm, 130mm, 150mm and 200mm being common.
I would suspect 114mm for a 900mm releflector - good enough to see the bands on Jupiter and its moons, as well as Saturn's rings - don't expect Hubble-like performance, and it will be pretty much black and white. Don't try to rack up the magnification too much , it will just make it more blurry in the end (I believe you said you had a variable eyepiece?).
It will be much harder to see any deep sky objects, especially in a light polluted region.
Can you show a picture of it, or its name ? - perhaps similar to this
https://www.maplin.co.uk/national-g...MIu7jqxu7X5AIVjLTtCh1suQ-FEAQYAyABEgKLa_D_BwE?
If it has an AZ mount, rather than an EQ one (basically, does it have a counter weight or not), then you cannot easily track things as they move as the Earth turns, but it is simple to keep a planet in the view, normally