Cannot disable a SIM card

[viennesewaltz]

I have a Samsung S20+ with dual SIM. In the Connections menu there is a feature called SIM Card Manager which enables you to disable one SIM and keep the other one on.  But this is not currently working for me.  Both SIMs are toggled to 'on', and when I try and toggle one of them to 'off' I receive an error message saying "Unable to change SIM card. Finish sending multimedia messages and try again'. I am not sending any multimedia messages.  I have force quit the Messages app, have restarted the phone and have reinserted both SIM cards, but the problem remains. Any ideas? Many thanks.

[Ventor]

@viennesewaltz Hello, maybe you should try to reset the SIM tools settings. Go to Applications and click on the area indicated in the screenshot. Activate: Show system apps, confirm and search for the SIM toolkit app. There are two-one for each card SIM In Storage, clear cache and data of both and check again.

[viennesewaltz]

Thanks, good suggestion but clearing cache & data of those two didn't make any difference unfortunately.

[Axoycan]

Hey, did you find a solution to this? I have exactly the same problem. My phone's 4G connection drops too. As soon as i remove the second simcard it get backs to normal. I get the same message as you when i try to disable one of the simcards on simcard manager... it is driving me nuts!

[viennesewaltz]

Hey, no I didn't find a solution unfortunately.  You would have thought the above suggestion to clear cache & data of Sim Toolkit would work, but it made no difference. And yes, my 4G dropped out as well.

[Axoycan · Accepted solution]

so after few attempts i got it working. i first deleted all sms from messages app, went into sim messages and deleted them too. then from messages app info i went into set as default then clear defaults. 
after this, it wasn't still working...
Later i removed the sim card tray then i powered down the phone, pressed volume down and power button simultaneously. phone restarted a couple of times after SAMSUNG logo. Before phone rebooted i powered down then did volume up and power button this time. i initially wanted to boot into recovery mode to wipe cache which i couldn't 

after reboot, switched sim 1 to sim 2 and pushed back the tray in and voila it was both working...I seriously have no idea which of the above steps worked but i got it working after 3 months...