Extracting certain frequency band

I have a signal that runs for 1 year. I'm only interested on the signal with frequency within 0.01 to 0.1 Hz. How do I extract this range of signal? I have tried several methods but I'm not sure if I did it correctly. For instance, I have tried using a function from this particular user (link: https://www.mathworks.com/matlabcentral/fileexchange/25017-fft-filter-clean-your-signals-and-display-results-) but the output doesn't seem right (refer picture). The "filtered" signal looks too weak and it's even symmetrical. I don't think that's possible since the original signal is anything but left-right symmetrical. Please help me.

Thanks!

Answer 1

[Walter Roberson]

Use a bandpass filter; see https://www.mathworks.com/help/dsp/ref/fdesign.bandpass.html

[Adib Yusof]

I will look up to the manual. Will be back to you once I've tried it. Thanks!