No connection with external monitor

Hi all!

Recently, I bought a 34 inch, Ultrawide monitor from IIyama to use with my Macbook Pro 16' (i9, 16gb RAM, AMD Radeon pro 5500M). When I first tried to connect it to my Macbook, it wouldn't show the signal, although my MacBook recognised the monitor.

After some calling with the webshop I bought it from, I decided to buy a dongle from satechi (which had a 4k HMDI connector) and it still didn't work on full resolution (only 2560x1440 instead of 3440x1440). So after another try with my own cable (HDMI to USB-c Thunderbolt from Native Union, which has HDMI 2.0) it sort of worked. Again, my MacBook recognised the monitor, but the monitor was black.

The problem was that I would have to change the refresh rate from 75 to 60hz for the monitor to shown the signal... And the signal sometimes worked and sometimes It didn't (it mostly didn't) The webshop said it had to do with my laptop 'being a Apple laptop and some monitors don't work well with them'. Later, I asked my local apple retailer later and he said that he had never heard of that. So I was very confused and decided to return the monitor.

Before I wan't to buy another Ultrawide monitor, I would like to ask what I can do to not have the same problem again. Do I need another cable, like DisplayPort to USB-C thunderbolt? or wouldn't that work either? Or was it the monitor? (although it did work fine on with my Windows PC with a standard HMDI-cable)

Thank you in advance!