@Bizin 

Hi,

May I know the model number of your new router?

After setup via tplinkrepeater.net, would the RE and signal lights lit up?

Have you tried to forget the router profile and reconnect to confirm that the password is not the problem?

Thanks a lot.

@Bizin 

Hi, thanks for your kind reply.

How about the model of the new router, not the 850re range extender?

And how long have you had the 850re?

If possible, please try to set it up under settings, such as

Please make sure the password is correct and there is no access control/Mac filtering on the new router!

Then check again whether the RE and signal lights lit up or not;

Thank you very much.

@Bizin 

Thank you very much and have you seen this link before:

TL-WA850RE failed to connect to my Hitron Modem: Disable Band Steering function on Hitron modem

https://community.tp-link.com/en/home/forum/topic/210886

Thanks a lot.