They should be signed as Interprovincial.
Its a bit more complicated than that. The National Highway System is a program in which the federal government offers funds to provincial (and in Ontario's case, provincial and regional) roads to bolster a properly maintained road network across the country. Unlike the US however, I don't really see a reason why we should have an "Interstate" type system. In the US, interstates make sense especially in the east coast where states are incredibly small, and highways constantly changing numbers is a serious issue. In Rhode Island, the section of the i95 in Rhode Island is a measly 70km, that is the distance of Highway 400 from Black Creek Drive to Mapleview Drive (the first exit in Barrie). It is shorter than the distance many people commute here in Toronto, let alone the distance people drive just to reach their cottages during the weekend. Having unified numbers in the eastern United States is kind of important.
This really doesn't apply to Canada. Outside of satisfying some internal OCD, the only location that would benefit from having more consistent signage and numbering is, what, New Brunswick and Nova Scotia?
Provinces like Ontario are extremely large, large to the point that what a highway is called in relation to another province is pretty much insignificant, and for those who live on the borders that would be impacted by it, they probably already know the highways by heart. There really isn't a pressing need to have a unified highway system, and I think that's okay.
One of the things I found very interesting reading through back in the day is how the highway numbering system works in each of the provinces, and its fascinating how much each province's numbering system speaks about the province as a whole. Quebec and Ontario (and also Nova Scotia I guess) are the only ones that have explicitly defined highway numbers for their major arterials, and shows how massive they are as provinces where that's something they have to explicitly declare, and even the way they handle it is quite unique. Quebec is extremely structured with a purpose and meaning to their numbers, meanwhile Ontario's 400 series highways are numbered by complete random without any form of logic or consistency to back it up. However, this system also shows how stringent both provinces are about standards. In Quebec, many of their autoroutes are actually 2 lane country roads where the only thing separating them from regular country roads are the fact that all intersections are grade separated (but even then there are some at grade railroad crossings), and the opposite is true such as the A20 which has traffic signals on Ile-Perrot. Meanwhile all of the 400 series highways have to follow a specific design standard of separated carriageways for each direction and fully grade separate crossings. The designation of "400-series highway" actually means something in Ontario, and we choose to follow strict standards with our highways, and it shows with how we have one of the most elaborate and highest quality standards when it comes to road construction and maintenance on the continent.
BC wanted to have a system that separated its major freeways from its regular highways, but it was removed after many of its highway plans were cancelled, which is quite indicative of that province, and more specifically the Greater Vancouver Area. Nova Scotia has a special demarcation for its major trunk arterials, but the standards for those are even lower than Quebec where at grade crossings in 2 lane arterial sections are permitted, and are still marked as "exits", which really signifies NS desperately trying to be seen as a cool kid on the provincial scene. Other provinces are more modest. Saskatchewan numbers their routes based off where they go. 200 series highways go to recreational areas, 300 series highways go to minor villages, 100 series highways are northern highways that are usually low quality feeders to the core highways that are 1-99. Ontario is the only province with a regional road system, which puts in full display how much autonomy we give to our municipalities, and our regions/counties.
This type of uniqueness is one of the things I personally find extremely charming about Canada, especially when you consider that its ultimately a federalized state where each province has its own history, background, and culture, and something as simple as their highway systems speaks a lot about them in their own special ways, and I don't think that should ever go away.