By fix mortgage, do you mean a fixed rate mortgage? If that’s the case, nothing changes because the interest rate is fixed regardless of any changes to property values, interest or the economy in general. This is a big reason many investors will tell you to look for cash flow instead of appreciate. Regardless of what happens to the FMV of the property, you’ll continue to cash flow positive.
If you buy a property for $100,000, rent it for $2,000/month, and pay $1,000/mo in expenses (including the mortgage), you earn $1,000/month in profit.
If the real estate market crashes and suddenly your property is only worth $50,000, and you can still rent it for $2,000/month, and pay $1,000/mo in expenses — then the value doesn’t matter (as long as you don’t want to sell it). Even if your mortgage I was for $70k and now you have a $70k mortgage on a property worth $50k, it’s okay because you’re still getting that cash flow, and after a few years pass, history has taught us that property values will rise again.
And even if the market dips, the property is now only worth $50k, and you can only collect $1,500 in rent, you’ll still be able to cover the $1,000/mo expenses and collect a profit. Your mortgage payment won’t change unless you refinance.