Right, the issue is going to be that you are in Singapore.
Zigbee is the same throughout the world, but Z wave is restricted to specific frequencies in different regions in order to avoid interfering with ambulance and mobile communications, which are also on different frequencies.
Singapore’s Zwave frequency is not the one that is used for the US, and it’s not the one that is used for the EU and China. Instead, it is the same as the one which is used for South Korea and Thailand.
All of which means you are limited in the devices you can use, as you can see from the list of compatible devices for a SmartThings hub which is legal to operate in Singapore:
I had some devices in mind for this project, but they are not going to work in Singapore.
Also the aeotec doorbell button is unusual in that it itself is not zwave. It is an accessory to their Z wave doorbell chime and only works directly with that device.
So you may be back to having to choose between the smartthings button or one of the Eria remotes. Which might mean having to put another Zigbee device as a repeater.
I don’t know, I Will have to think about this a little. Hopefully some other community members will have additional ideas.