#4: The lost valley of the Hutaakan in Karameikos.
And none of these suspend your disbelief? That's just some examples to show it isn't hard for a valley like TR to exist in the well known areas of Mystara.
It's a matter of internal coherence, not scientific possibility. The Hollow World is unknown and isolated for specific reasons until in canon some events happen to make it known. #4 and TR are more problematic because they exist in the same general region and I know, from real world history, that isolated communities in general are not so much isolated, or not for long anyway, certainly not for centuries, let alone millenia.
At some point, a goat herder or a hunter will see the valley, a trader will reach it, unless there is some magical explanation why they cannot.
Also even simple societies are more dependant on trade with far away places than most people think, unless they are really at a basic stone age level. But a medieval-like society? Where do they find metals, clothings, food? It is very difficult for a small society to survive isolated. You can decide it is so, but then you have to assume they have limited choice of food and materials, they recycle everything and luxury items have very high price, if they are possible at all. Where do they culture come from? Why certain races or monsters have not been exterminated yet? Why people do not try to go over the mountains?
You can also explain everything with a shrug, magic, the will of the immortal, or else, but I prefer to have some degree of plausibility, and a valley isolated for centuries, yet with a material and immaterial culture similar to the Know World countries, certainly does not make sense to me.
While Hutaaka may have a reason for its isolation, TR has not, being just a generic mini setting.
I think a DM who wants to connect TR to the rest of the world has to work a bit on it to make it work.
That's obviously only if you want to give it a life beyond the run of its modules.
The Hollow World also could be an idea, but then you should find a reason why such cultures where preserved and from where in the OW they came.
KK however is just a carbon copy of current (1000 AC) Karameikos events with different names so there is really no point in placing it in the HW..