I have the same problem as described in these posts where the clients (max 50 machines) face "DNS not resolved" error in Chrome, all of a sudden after 10-15 mins due to DHCP crashing and not resolving DNS.

https://community.tp-link.com/en/business/forum/topic/1402

https://community.tp-link.com/en/business/forum/topic/273652

I tried to disable all firewall attack defense options but still the problem prevails.

The only way I can resolve this problem is to disable the DHCP in the router and run a separate DHCP server.

Firmware: 

3.0.2 Build 20200313 Rel.47458

Hardware:

TL ER6120 v3

While the issue is resolved by running another dhcp, I still would like to know why can't this router handle 50 clients simultaneously?