That seems to be impossible. How do you do the test? If you truly have issue, you could contact TP-Link support or you could replace the new one from seller.

Whether if your modem could assign different ip address for your network and do NAT? If not, you also need a router.

If you don't use small patch panel, just connected two switch by cable directly could they work? You could also do the cable test in switch web page. Or change another ethernet cable.

I didn't find that. Could you share the link to me?

There is a beta firmware i get from TP-Link support team. And it seems to limit the speed of arp packets. Here is the download link:...

Could you describe more clear about 'Nothing can see internet'? Whether if you could get ip address for wired device? TL-SG108 is just an unmanaged switch, if it cannot still work. You could test...