@Phili 

Deco itself can be your main router. But since you've already bought AX50, you can follow this guide to utilize all the hardware you have.

@Phili 

You configure AX50 normally as a wireless router, nothing special should be set there.

According to the guide I posted, if possible I would prefer ethernet backhaul - that will guarantee best speeds between S4s and its wireless clients - more here.

Whichever scenario you choose you should observe the AX50 ports negotiated speed to be sure everything is correct:

If Internet port is lower than 1000 mbps and your connection to the ISP is 1000 mbps then change this setting:

If the issue persists use other quality ethernet cables Cat 5E/6. Observe the LAN port where S4 is connected too.

To test if everything is OK plug your computer with an ethernet cable to the AX50 and test the speed.

Then plug your computer with an ethernet cable to each S4 to test the speed again. That way you can find where the problem is.

You may have put S4s too far from each other if you've chosen wireless backhaul scenario - use this guide for placing them.

The use this guide to get to know your Deco app.

Or check your AX50 and S4 wireless settings, I would recommend these:

Differences between router mode ans AP mode on the Deco here.

Also make sure that QoS feature is disabled on all devices.