Formula to select an exchange rate based on currency and date of payment

Hi everyone,

New day, new Ninox puzzle :)

I have a series of fields in a subtable recording past exchange rates for a series of currencies (we use InforEuro, so these are monthly exchange rates). I would like to use these rates in different places across the table, including to automatically select the appropriate exchange rate from other currencies to CHF in my payment tracking tool. I need a formula that, based on the currency and date of payment, picks the appropriate value across all the currencies and months recorded in my subtable.

For some reason, however, I can't even manage to accomplish the first step of the formula creation. I have a formula field; if I select the relevant subtable field 'Exchange rates'.'EUR->CHF 2019/01' the formula field just remains blank. What am I missing? EUR->CHF 2019/01 is a simple number field.

Once this is solved, I have an idea of how the formula could work but I wonder if it could be optimized. Something like this:

if 'Donor Currency' = "1" then
if 'Date of receipt #9' > date(2019, 1, 1) and < date(2019, 1, 31) then
'Exchange rates'.'EUR->CHF 2019/01'
else
if 'Date of receipt #9' > date(2019, 2, 1) and < date(2019, 2, 28) then
'Exchange rates'.'EUR->CHF 2019/02'
else
if 'Donor Currency' = "2" then...

All suggestions welcomed and appreciated.

Best,

Giovanni