Bus 315/317 to Serangoon Interchange (MRT/Bus), 20 min
Bus 73 to Ang Mo Kio Interchange (MRT/Bus), 25 min
From the MRT stations above, you can proceed to the CBD. Travel times may be about 1 hour including walking/waiting time.
For express bus
https://landtransportguru.net/bus534/
534 passes through Serangoon garden way but I am not sure if the destination/route is convenient for you.
I do not know if they are operating at this time due to COVID CB.