PS4 drastic connection speed drop recently

Before I start, YES, I know that there are 101 pages of similar problems and similar error, but no clear steps are given, nor explanations. So:

We have had our PS4 for 6 months, and in that time i have occasionally checked internet speeds. Our modems average, via Ookla Speed Test, is ~14Mb/s down and ~750Kb/s up. The PS4 network test (Again I will save myself pain: I know it is unreliable, I simply compare it to itself) usually comes up with ~10Mb/s down and ~650Kb/s up.

Now, that was until recently. A few weeks back, a local server went down (Telstra is our ISP, an Australian one for anyone who doesn't know), and we had no internet for 24 hours. Since sometime between then and now, the PS4 speed has dropped to ~2.5Mb/s down although ~700Kb/s up – yes, that has apparently increased. We now get irritatingly persistent lag issues, and while still playable we are constantly reminded by Battlefront of our 'weak connection' in the top right of the screen. However it is important to note that the Ookla Speed Test still gives the same results, so it is our PS4.

Notes:

• During the PlayStation network test, it occasionally says we have NAT type 3, rather than 2 (only rarely)
• It claims that our router doesn't support IP fragmentation, which it never used to say.
• We use wireless rather than cable, ethernet isn't available (only one port in the whole house).
• We're in Australia if you couldn't tell, which is why 14 is actually a great speed for us – Australia has shocking speeds.

Attempted Fixes:

• Changed DNS servers to 8.8.8.8 and 8.8.8.4, didn't change anything
• Changed PS4 MTU size to 1470, incredibly fractional increase in DL speed
• Changed router MTU to 1500 and 1450, neither helped.
• Something perhaps important: our 2.4/5Ghz networks were both set to half their maximum bandwidth, but increasing them to full did nothing
• For the IT Crowd fans, yes I tried turning it off and on again, both the PS4 and router.

What I would like is a succinct answer, with (preferably) every possible unmentioned fix in order of likely effectiveness, and why it is done – mostly because every other page seems to be a reddit page which requires an hour of reading with no benefit, and I feel like there should be a good list of options out there for everyone. Thank you in advance.

AND

if this is the wrong site for this question please just tell me rather than reporting it, I wasn't sure which to use.