@gumpdeco88 

It is possible that your Main Deco negotiated 100Mbps speed over wired link to ISP modem/router. Try different Ethernet cable.

Also check Deco app, if you see Abnormal WAN Port Speed message in it. If you do, this confirms your Main Deco can't establish gigabit link speed over Ethernet cable.

  @Alexandre. Thanks for the response.

the Ethernet cable is from the service provider directly and I do not expect gigabit speeds here since the max bandwidth I have paid for is 200 Mbps. 
 

It was working before recent firmware update and not sure if firmware is causing the issue. 

  @gumpdeco88 

When two networking devices, such as Main Deco and ISP modem/router connect by Ethernet cable, they negotiate discrete Ethernet speed between them. They can either agree on 100Mbps or on gigabit.

If Main Deco negotiates gigabit with modem/router, you will see speed test results reaching 200Mbps: it is what your ISP sells you and it is below maximum speed of a link between Main Deco and modem/router.

When Main Deco can't negotiate gigabit speed with modem/router, they will agree on 100Mbps. this is what is going to limit speed tests results.

There may be other reasons for drop in speed, but when it runs very close to 100Mbps, not more and not much less, the first suspect should be Ethernet cable between Main Deco and modem/router.

It is also possible that firmware update caused this, but indirectly. It may have become more sensitive to Ethernet cable issues and could more aggresively downgrade speed to 100Mbps if there are errors in data transmission over Ethernet cable at gigabit speeds.

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In summary: before trying anything else, try different Ethernet cable between Main Deco and modem/router.