First of all, if freezing and or heating works for your iPhone, it simply means the WIFI hardware in your iphone is NOT defective.
What is defective is likely caused by OVERLOADING by the new IOS, which is not designed with your oldie in mind.
Good luck to those who managed to revive the WIFI for long by merely hot blowing with hair dryer, if my experience is anything to go by, such low level fixing can only last so long; Thereafter you may have to repeat the same thing, and to do it harder and harder before the WIFI eventually dies perpetually.
Remember freezing works is because there is a safety shutoff to prevent overheating , this is never the cause of no WIFI. Heats works is because it opens up the "bottleneck" a little more to allow the demang new IOS to pump massive data through, but such filmsy fixing is not likely permanent. Over time, it should break down, and count yourself lucky if it can last for more than a week.
So, what is the solution ? Give Apple at least $200, and I am sure Apple is confident you will give them the money to clear their old stock. Their cost is probably only $50 according guru that strip gadgets apart for cost analysis.
Why ? Because you can't get a comparable Android for $200.
Now to make you upgrade IOS and don't allow you to downgrade simply suggests making consumers buying new phones is what matters to the Apple. And as such, there is probably no way out unless someone can convert your IOS back to the good old ones or perhaps repair shop is able to find a an easy way to bypass the "weak" connection without charging you an arm and a leg, Which is not linkely because I imagine they gotta pop open your phone yo either physically heat/solder something or change cable/pin etc.
Remember, never ever upgrade software on all Apple products because no one can help you when faced with similar problems. Apple has make it very clear they are not interested in oldies. They want you to buy a new hardware every 1-2 years. They're big, they have to show some numbers to Wall Street and unlike the old days, they are bankrupt of idea ... they're still selling the same old iPhone.