@CommanderZar 

Hi, without the EasyMesh ethernet backhaul, you could configure the second BE550 into AP mode(Case 1): How to configure the TP-Link wireless router as Access Point

For the constant drops issue, there is no known issues, do you mean it happens with 5GHz WiFi connection only? will it disconnect completely at the same time on all 5GHz client devices or it still shows connected but no internet access? and will it recover automatically?

You could try disabling smart connect and change the 5GHz WiFi channel and channel width on Advanced->Wireless->Wireless Settings page to see if it will be more stable.