As far as I know there's no indicator in the Deco software as to what link speed the ports are operating at.  That said, any speed over 100Mb means it has to be operating at 1Gb, as your only choices are 10M, 100M, and 1G.  If the router's speed drop only recovers on a restart, it is likely to be a issue between the router and your ISP - I am presuming that the router you refer to is ISP supplied kit?  If so it's probably best to contact them.

By the way, port speeds are in bits per second, not bytes - could be just a typo, but some speed tests measure in bytes and mixing them up can make you think your speed is much slower than it really is :-)

Is it the speed you get from the Deco M9 plus or it is the speedtest results on Deco app? 

If it is the speed test on Deco app, connect your computer to the modem and main deco by wired cable to do a comparison test, if they are the same, the speed should be fine. 

While if it is the actual speed, first ensure that you can get normal speed from the main deco; and try to check the wired/wireless link speed when the speed slows down: 

Meanwhile, you can set QoS as standard. Open Deco app, go to more-Qos.
And clear all high priority. In QoS page you can see device priority, click high priority and then you can clear them.

@1000bb I have exactly the same issue. My deco p7 after every several days switches from 1000M to 100M WAN port speed. The only thing which helps is turning off and then turning the deco on again. It's frustrating. I can't see any way of enforcing deco to stay on 1000M mode. I asked the ISP kit provider, but he has said that the Deco device announces up to 100M speed capability for whatever reason and they can't change it.

Any idea how the issue can be solved?

@TP-Link in short that would be my network topology:

Speed test was done in deco mobile app. I assume it uses main deco device (P7 main)

The frequency of the issue is every few / several days.

@meceo 

You should be able to check what port speed Deco P7 negotiated with ZTE F680, by visiting ZTE F680 Ethernet status screen. 

@Alexandre. Thank you for the hint. I already know that deco P7 initially negotiates 1000M, then in few/several days renegotiate to 100M. The entire point is why and how to prevent that. Both devices clearly are capable of doing 1000M as that is the initial speed.

@TP-Link deco p7 is in wireless route mode. The cable wasn't changed. However, I don't remember what kind of cable is it.