Hello @magbabalot
If Range extender does not detect router's Wi-Fi signal, you could try some tips here:
What should I do if the range extender cannot find my host network?
TL-WA855RE is a single band Wi-Fi range extender which supports 2.4Ghz Wi-Fi band, not 5Ghz band
Confirm router 2.4Ghz Wi-Fi band is available for other network devices , if its a dual-band wifi router, try to use different SSIDs for 2.4Ghz/5Ghz wifi bands
Try changing a different 2.4Ghz Wi-Fi channel on the main router, channel among 1-11 should be fine.
If still the same, please share 1) a photo of the router 2.4Ghz Wi-Fi settings 2) a photo of RE scanning Wi-Fi page 3) could range extender see other Wi-Fi networks ?