@Kevin_Z 

Hello and thank you for your quick reply.

The RE450 operates in Range Extender mode. I am certain, that the connection between the RE450 and my OnePlus is stable, because the only thing I changed in my setup is the AX50. Before I had a WiFi 5 router, with which everything worked fine. After changing it for the AX50, the trouble started.

The AX50 is working as a Router, but I will try changing it to AP mode.

@schmidi000 

If Dynamic IP doesn't work properly for some reason, then try if it works better if you set the internet connection on the AX50 to Static IP.

As I take it, in your setup the AX50 is the only client device that connects to the RE450 anyway.

Most (if not all) Android devices use the IP range 192.168.43.xxx for the Wireless Hotspot.

On the AX50 in router mode set the Static IP address to something like 192.168.43.200, Subnet Mask to 255.255.255.0, the Gateway IP to 192.168.43.1 and the DNS IP to either 192.168.43.1 or any third party DNS you like.

@schmidi000 I learned this tip when putting on a VPN on my Asus' (plural) routers.  I kept losing internet after an hour.  I was freaking out.  The free DNS services didn't help.  (Cloudflare, etc.).

HERE's WHAT HELPED (it fixed my problems):  (and I think it's a unversal fix, and apparently it's well known in the VPN community).  It can't hurt to try.  

Go to the computers you are using to access the router admin (mine is on an ethernet cable, but I don't think it matters).

I only have windows, so that is where I'm coming from.    

1.  Open a CMD (command prompt).  

2.  You get a black screen 

3.  type this exactly: ipconfig  /flushdns    (you should get a notice that the "resolver has been flushed"

4.  (wait)

5.  type this exactly: ipconfig /registerdns   (you may get some grief about not being the admin, or the local media is disconnected, but it doesn't seem to matter 

6.  Now try your DNS on your system (cloudflare, Quad9, whichever).  

This completely FIXED my problems.  What seems to me to be occurring is that all the 'junk' DNS' from time past are causing your router to lose internet. 

There IS a MAC (Apple) fix:  

Google:  clearing DNS on a MAC.  

I also found this:  in the stories:    "check the lease time of the "DCHP" (in the users' case it was set to 5 minutes, and causing the crash).   My ISP assigns a time of 2 hours.   

Good luck!