Most smartphones only support DSDS (Dual SIM Dual Standby) not DSDA (Dual SIM Dual Active).
Edit: Qualcomm has now resurrected support for DSDA in case of SD 8 G2 and SD 7+ G2. However like earlier it's upto the device manufacturer to actually enable it. SD845 supported DSDA but while Oneplus enabled it later with a software update in Oneplus 6 series, Xiaomi didn't in case of my Poco F1.