So I’ve managed to edit the tablet and phone viewpoint however, if i change to a different phone setting, the layout gets messed up because of the different height of the phones. So I’m planning to make a first page warning where it gives a pop up mentioning the website is best viewed on desktops and tablets but the issue right now is, if i make a page on the phone view point, it will reflect on both desktop and tablet view point which makes it very annoying cus i only want the first pop up page to appear on the phone view point. Is there a way to choose the default page for each view point?? thank you! 
you could stay with your original pagesetup, but in the mobileview version you could create a popup with your informationtext and a little closebutton. (shape as background + text + some shape to close it). To all of the 3 elements add an animation “on click” with the closebutton as trigger and let them fade out.
make it “above pages” so if someone will directly visit subpage of your website the popup will also appear.