@Daniellucas issues like this can be caused by many factors,  Such as power outages, power surges, issues with the router or issues with the range extender itself.  The 1st step in troubleshooting this would be to power cycle the device by unplugging for 30 seconds and then reconnecting it and seeing if it restores service on that band.  If not the next step would be to check if you are on the latest frimware.  for the RE355 our latest frimware is:  https://static.tp-link.com/2018/201803/20180307/RE355_V1_180204.zip.  Once updated then test again.  if still down the next step is to hard reset the device and run through the setup again.  during this step its important to see if the default name is showing for the 5GHz band and if you can connect and access it.  If you are able to then this indicates the issue is located on the 5GHz band of the router, which could be channel or channel width related or require more troubleshooting with the router's maker.

Let us know if this helps

Thank you 

Carl | TP-Link support 

@Daniellucas Thank you for the screen shots.  So I have a few more questions with this infromation.

1.  When you defaulted the RE where you able to connect to the default 5GHz band and log into the RE's UI page?

2.  If so were you able to successfully configure the RE?

If you were it would appear there is possibly an issue in the signal reaching the RE.  This can sometimes happen if another network is close and operating on the same channel creating interference.  This interference can cause a single to be reduced even though it onced worked.  A few things I would change on your router. 

1.  The bandwidth.  Change it to 40MHz or 20MHz.  the best way to describe what this does is a Mag Lite Flashlight.  80 is when the flashlight head is twisted loose and although it covers a bigger area the light is dimmer and doesn't go as far.  As you tighten the head the beam gets brighter and more narrow, providing a stronger beam further away.  The smaller the MHz the more narrow and more powerful the single but it doesn't cover as great of an area. 

2.  The channel.  I would recommend shifting the channel to the upper level chanels like Change 153 just to see if it helps.

Beyond this I would update your router's Firmware if not already done.  If there is still an issue we can replace as long as the unit is still under warranty, as there is always  a possiblity the unit simply went bad.  Let me know how the above test goes and we can go on from there. 

Best Regards

Carl | TP-Link Support 

Glad to hear it.  I hope the extender continues to operate as expected, but let us know if you have any further questions

Best Regards

Carl | TP-Link Support 

@Chelsey 

I only have a number that can be used for US customers.  Based on your regestration location you are in Spain.  As support is regionalized you would need to https://www.tp-link.com/es/support/contact-technical-support/#LiveChat-Support  for your region's methods of contacting a live agent.