Mobile Network Not available

I have a circle with a line through it where there should be cellular signal strength. Have tried all the “ususal suspects”, multiple network resets, wifi calling is off, reset phone to factory defaults and even full factory reset and reload software. Tried multiple sim cards, all of which work in other phones. Phone is now apart…..all parts look in tact and fully functional. Phone seems to recognize when I install a sim card, but never gives a connection to a network. From phone firing up after complete system restore, the circle with the line is present. Any ideas?