I have the same question. I am repeating a 2.4ghz signal but RE200 extended it to another channel which is very busy. On the advanced configuration page we do not have the option to select the wifi channel.

Hello @Javielfo. I did some tests and i found this:
If you are talking about 2,4Ghz band when you connect to 5Ghz and the repeater extend 2.4 Ghz. It will automatically use a different channel that your currently 2.4Ghz band is.
For example:
Case 1: You have a Dual band router (Channel 36 and 11) and connect the repeater to 5Ghz, The repeater will extend the signal on channel 1 or 6.
Case 2: You have a Dual band router (Channel 36 and 11) and connect the repeater to 2.4Ghz. The repeater will extend the signal on channel same channel 11.

Unfortunately i can't find how to change the channel manually on RE200. If anyone knows it. please let us know.