@MattNegev 

At a quick guess the router your ISP provided is a WiFi 6 device, whereas the M5 is a WiFi 5 device.  Basically the router is a newer version of WiFi and therefore will offer a bit more performance

WiFi 5 speed tops out around 500ish..  WiFi 6 can push this a bit higher to around 800

You could try running network optimiser on the app to see if a new channel will help, however 520/550 is your limit for the M5.   You would need a deco starting X (x20 for example) to get the faster speed 

  @Philbert but the speed I get is from deco speed test so it's not going through the wifi. It's going directly through the network cable. 

The high speed I measured before was with the deco set as dynamic ip and connected to the isp router and I would be happy if I could get that speed

  @MattNegev 

Ah ok that changes things..

To be totally honest, I wouldnt put too much faith in the Deco speedtest, always found it to be wrong and I have used the deco hardware in Dozens of installs.

You would be better checking this via something like   FAST  DOT COM_    or   SPEEDTEST DOT NET_   (sorry for typing, trying to pass filter, hope makes sense!).   They tend to be a lot more reliable, the deco is really only to prove you are online and not a quality speed tester, its inaccuracy has been noted a few times on the forums. 

What is your line speed supposed to be?   Generally hardware doesnt make any real difference to the WAN speed, time of day / contention etc can however.     I would repeat your testing on a wired laptop plugged into the Deco on FAST DOT COM_ and see how that looks in comparision to what you are supposed to be getting.   

For the record, i have a 1gb line and its anything between 700 and 950 at various times of day.   If you repeat the test hourly you will notice massive differences so best ruling this out first before going for a deco hardware problem.

  @Philbert My speed is 600/60

 I just has the isp technician here and we tested the speed in two ways

1. with there router connected to the ont bridge and my deco connected to their router with internet connection type set as dynamic ip. and got great internet speed of 610/69

2. deco connected to ont bridge with deco's  with internet connection type set as PPPoE 508/67.8 sometimes 465/67

It seems to me that the deco cannot get the maximum speed when it's handling the internet connection