How reliable is the validation process for the time series analysis?

Please take a look at week 1 lectures and notebooks. Concepts such as seasonality and differencing (a good strategy to address trend & seasonality) are described there.

autocorrelation will give you a guideline on how far behind to look for differencing.

Read this to understand the importance of normalization.

I recommend you go till week 4 since that’s where you start using conv and recurrent layers for modeling. See if that helps as well.