SD card no longer recognized

I have a Samsung Galaxy A425G, my wife has an A14e (?). We switched our phones to T-mobile last year and I purchased 2 SanDisk SD cards, inserted each properly, was not prompted to format the disk on either phone and when I run diagnostics, neither phone recognizes the SD cards.  Except for occasional connectivity issues, we've had no other issues. Are SD cards absolutely necessary for full phone operation and capabilities?

Thank you!