Using IN operator OData 4.0 "Bad Request" FetchXML Builder xrmtoolbox

Question

By using FetchXML Builder in Xrmtoolbox I have an OData 4.0 string:

.../contacts?$filter=(searchable eq true) 
             or (contains(firstname, '%25firstName%25)' 
             or contains(lastname, '%25lastName%25') 
             or Microsoft.Dynamics.CRM.In(PropertyName= '_accountid_value', PropertyValues=['00000000-0000-0000-0000-000000000000','c198b30d-8d4e-eb11-a812-000d3ab5acdf','6c9caaac-a755-eb11-a812-0022489b9c9c']) 
             or Microsoft.Dynamics.CRM.In(PropertyName= 'contactid', PropertyValues=['00000000-0000-0000-0000-000000000000','30262317-0a83-eb11-a812-000d3ab02826']))

I have manually changed "and" to "or" in the query, so I am afraid it is just a small detail that I have missed. However, I have never used the IN operator with this tool before and am therefore worried it might be an issue with the way I use it/I have done something wrongly.

How do I know there is an issue with the string? The image below shows the code in C# when I am trying to fetch the data. This is where I can't get through because I get "Bad request" from the HTTPClient.GetAsync method. I know that the method call is not the problem because I do this in several other places, and can therefore only assume it is the OData string. I have also got "Bad request" before and fixed it making the OData string correct.

Has anyone else had this issue or know what is wrong with the OData string that I am overlooking?

Edit: I checked the docs linked by @PanagiotisKanavos. Unfortunately this also gives my "Bad request".

".../contacts?$filter=(searchable eq true) or (contains(firstname, '%25Mat%25)' or contains(lastname, '%25son%25') or _accountid_value in ('c198b30d-8d4e-eb11-a812-000d3ab5acdf','6c9caaac-a755-eb11-a812-0022489b9c9c') or contactid in ('30262317-0a83-eb11-a812-000d3ab02826'))"