You should always try to match connectors 1:1, or if you have no other option but to mix and match, you should always provide a connector/cable/converter capable of delivering more bandwidth than necessary to your display. GPU HDMI to Display DP may not work if your card's HDMI jack isn't 2.0. Previous HDMI connectors/types (pre-2.0) can't deliver the signal bandwidth necessary to fulfill the demands of HDMI 2.0 and DP resolutions/refresh rates. So if you're running GPU HDMI --> Display DP, that may be why one of your displays isn't working - it isn't getting enough signal bandwidth to power its display through the connector you're using. It has nothing to do with your monitors not all being the same model, lots of people mix and match resolutions/refresh rates, including myself.
So in essence, yes, it does matter which port you connect to, because pre-2.0 HDMI can't provide enough power to drive high-bandwidth DP configurations such as 1080p 144hz, 1440p 144hz, 4k 60hz etc. Mind providing us with more information regarding which connectors/cables you're using to drive each monitor?
Now I know for fact that the 1070 has HDMI 2.0, so I'm not sure, but perhaps your cable has to do with the problem here. I remember certain HDMI cables from 8-10 years ago would support different resolutions, such as some only supporting up to 720p 60hz and others supporting up to 1080p 60hz. I'd do some research but I need to get back to working on some assignments for uni.
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