The mere fact that it works fine on your iPhone already rules out a problem with your SIM card roaming profile. Your Poco phone may have done some updating which messed up with the roaming. Android is just the base system software, the hardware manufacturer does a lot of configuration to cater to different component drivers that are used inside the phone. You can write and complain on Simba's FB page, they have more than 800k subscribers which is 10% of the Singapore market, at any point in time, there should be hundreds of users in China roaming, so if this affects Android users, they probably are flooded with similar complaints or you can perhaps trigger similar complaints that will compel them to investigate further.