If you've not already done so, download the user manual:
https://org.downloadcenter.samsung.com/downloadfile/ContentsFile.aspx?CDSite=UNI_UK&OriginYN=N&ModelType=N&ModelName=SM-A125F%2FDSN&CttFileID=8135061&CDCttType=UM&VPath=UM%2F202105%2F20210513142713038%2FSM-A125F_UM_EU_RR_Eng_Rev.1.0_210513.pdf
Again, if you've not already done so, put the SIM card into your phone, as per the instructions on page 15 of the user manual.
Put the microSD card into the slot behind the the SIM card, as per page 16.
That should be all you need to do.
If you get another SIM, for a different network, (perhaps because one service is cheaper abroad than the other is, or something similar), put it into the spare SIM slot and use the SIM Card Manager (see p.17) to switch between the two networks.