Calculate Profit/Loss per Month, based on Data from another Sheet

Hi there,

I know there are some threads that exist covering this topic - and many online resources that try to guide people through the process - but I am struggling to get them to work/work in the way I want them to.

My aim is as described in the title: to create a formula to sum specific data metrics within columns - not set specific cell ranges - based on the month, (i.e. monthly sales total for October, 2021; or Postage Charged total for November, 2021).

This is because as sales continue to come in, I do not want to have to keep going editing the formula's for these metrics to extend the ranges, if at all possible.

Currently, I have been attempting to use a "SUMIF" function in tangent with an "EOMONTH" function, but I have not been able to get it to sum any values at all. I have attached my test sheet - with the current formula present in the "Reporting" sheet - with this post.

Please note:
- The calculations sheet is named "Reporting" and the data input sheet is named "Activity".
- Sheets named "Sales", "TM Account" and "Bank" are irrelevant to formula currently, and are there purely to keep track of records in the future.
- I have supplied several "dummy" transactions in the "Activity" folder to assist in testing and putting together a formula.

I would be very grateful for any assistance anyone can provide.

Thank you, stay safe, and have a lovely day.