Airport Express in client mode to serve Airport Extreme

Hi guys,

I'm struggling with the setup in our business that is rather peculiar. Our cash register, bank terminal and order system are all served by an Airport Extreme that was carefully configured by a dedicated company. The Extreme is fed via an Ethernet connection from the base modem. The base modem is a nondescript box from the internet provider in our country that provides four ethernet ports and a basic WIFI network that we open up for customers. As the base modem is too far from the Extreme, a sitecom powerline was originally used to transport the signal from the base modem to the Extreme.

But the powerline connection was poor so I was advised to replace the powerline by an Airport Express in client mode that converts the customer wifi from the base modem to a ethernet signal for the WAN port of the Extreme. I installed the Express exactly as outlined by Apple (AirPort Express: How to join an existing Wi-Fi network in client mode - Apple Support) and the whole setup ran smooth for about an hour. At a seemingly random point in time, all of a sudden the Extreme complains that the internet connection is lost. In airport utility, the Extreme blinks green, but the internet icon is amber and the notification says the connection is lost. However, the Express blinks green and is still connected to the WIFI that I confirmed was still working properly. Restarting the Extreme and Express solve the issue, but only temporarily. After a while, the Extreme complains again that the internet is lost.

Can somebody explain what might be causing this issue and how to solve it? Thanks in advance.