@pixelart 

I am getting 300Mbps according to speedtest, on my Android 9 smartphone connected by WiFi to Deco M5. M5 is wired to ISP modem/router. My Internet connection is 300Mbps down /20Mbps up. I run my Deco mesh in Access Point mode.

I saw recommendations on these forums to check and up QoS value for M5 running in router mode, in situations like yours. If nothing helps, consider running M5 in AP mode, at least for testing. 

@pixelart I get around 400Mbps (at times I got 500Mbps as well), so DECO M5 is not limited to 100Mbps. It is possible that QOS was set in your Deco with 100Mbps as speed limit? If I recall, there is no way to remove QOS once set but setting the values to very high is almost same as removing QOS! I use DECO M5 in AP mode though.

I recommend trying another Ethernet cable. In my case, even the one supplied with the Deco package was a faulty one and limited the internet speed to 100mbps. Using another cable, it works perfectly now.

@pixelart Thanks everyone for your replies.I finally got things working by reseting the main Deco unit with a pin. After running set up things started to work - I'm now getting 352 mbps.