Answer 1

Few this could be up

1. Start off with rebooting the tablet if you had not done so. Because tablets do not require a battery removal to allow access to the SIM slot, users normally take the advantage of changing the SIM cards while the tablets are powered on. While this is harmless, the device will sadly not register the SIM until a reboot.
2. Try a different SIM card if you had not done so (also try your SIM card onto a different phone) as SIM cards sadly goes bad at times.
3. Try a close visual inspection of the SIM slot - ensure you have all six gold SIM contacts in the slot (cleaning may be necessary if all six are there)
4. The SIM mechanism on the tablet's main PCB may be bad - you will have to get in touch with Sinoprima (the maker of Sinotec branded products).

Please do comment with any further necessary help on the issue.