Welcome to Android Central. We're going to need more answers to offer you the best advice.
Has this ever worked? Since you've had this device has it worked? Did this just start? How long ago did it stop? Did anything happen around that time?
With the Samsung's A series devices they do not have IP ratings, did this phone get wet at any point? Moisture could cause connection issues. With the SIM card tray being a "sealed" compartment I wouldn't expect to find debris in the slot but if debris is suspected, with the tray removed tip the device so that any debris would fall out and give it a few sharp thumps into your hand. If you're going to try compressed air, blow from one side of the opening so that air and hopefully the debris both get pushed out the other side. Blowing directly into the slot may push whatever deeper or cause it to be more stuck so be cautious with this method.
If there is no suspected moisture or debris, have you contacted your carrier? Even though the SIM works in other devices carriers get calls like this everyday and should have reps that can handle simple troubleshooting and recovery.
Without knowing if the device has ever worked for you, is this a new device? Where did you buy it? What did the listing say, if not purchased new? If you bought a defective used device you may consider attempting a return or reaching out to the seller for advice or help.
Samsung's are fairly universal but if not purchased from your carrier, have you checked that the device is compatible with your carrier? Devices are not arbitrarily denied from carrier lists. There is usually a compatibility issue or security issue that get devices excluded from carrier lists.
Let us know what you find.
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