Welcome to the Samsung UK and EU forum! If you go to Settings > Connections > SIM Manager > More SIM Settings, there will be an option for Dual SIM Always On. I hope this helps!
Please try a soft reboot by turning the phone Off and On.
If that does not help then also try Clear Cache Files from the phone's System Partition Section > https://www.samsung.com/uk/support/mobile-devices/how-do-i-clear-the-cache-on-my-device/ You may need to insert your earphones or hook upto a powered On Laptop or use a Tv's USB Port to access the Clear Partition Section. This may help with lag or stutters as this clears the phone of old/unused or broken cache files.There is also a Repair Apps option in the Menu System too { Possibly excluding the s²⁴ Series of phone's }
