R6250 Internet connectivity issue (slow speed) after firmware updates and an workaround

After updating the original firmware of my R6250 (at this moment this is the third update to R6250-V1.0.4.8_10.1.13) the internet port of my router  doesn't connect in 10/100 mode and now connects only in gigabit mode.

As result my internet sped drops to an unusable speed to 0.5Mb/s

The router is connected to my cable modem (Motorola Surfboard SB6141) that acknowledges this connection by changing the activity led from red (10/100Mbps) to blue(1Gbps).(the modem has Comcast Xfinity 25Mb/s internet service)

The workaround was to slice the Ethernet four twisted pair cable that connects the router to the modem and cut two pairs (in my case blue/white blue and brown/white brown pairs - pins 4,5,7,8 of the connector)

Now the router connects in 10/100 mode and my internet speed is back to 29Mb/s

Did anybody else had encounter this issue and is there a better solution?