("This computer got internet before I could afford a router". Dial-up, was that? I'm not sure what else (other than a historical T1/ISDN/whatever lines, and all those older solutions were essentially premium services anyway) you might have done without a router by some name or other.)
But, anyway... that MAC situation sounds odd to me, as well. MACs are supposed to be unique identifiers, so unless you have a different outward-facing one to the inward-facing one[1], it's possible that some of your basic problems are with OSI Layer 2 issues. (Although I'd be surprised it worked in the first place.)
And, moreover, every (consumer) device that I've dealt with tends to come with its own default MAC that is unique (vendor-provided), and yes you (or the ISP) can use that as a very basic "are you who you say you are" (although, given you can change it at all, often, it sucks as a guarantee that someone isn't deliberately spoofing a valid device[2]), but you should be able to tell ISP that /this/ is now the MAC you wish to access (after resetting the router so it's not lying and hopefully being honest about itself). Depending on how good your ISP is, this might mean a small amount of transition time while they and your device are working under different expectations.
I'm not sure if any of that will help the rest of your problems, though. Sounds like you just don't have a consistent 20 m/bit connection, in the first place, but a flaky one because your ISP/telco promised you something that the line to your house can't support (perhaps due to interference on the way from the exchange/fibre cabinet involved). And, just want to check... microfilters are being used accordingly? Probably not a problem, but I've seen that. Plus badly re-wired (and not telco-sanctioned) phone extensions, in one house. It can't generally hurt (unless impractical for other reasons) to have the router (filter-)plugged directly into the primary house socket.
(It's always possible that I'm speaking from experience with totally different technical set-ups from what's relevant to you, of course. Or just not doing a good job of explaining/understanding. Somebody will be along to correct me, I'm sure.
)
[1] Or you're conveying the inner one outwards in some form of 'bridging mode'. Some routers are highly configurable and let you do something like that.
[2] I've worked with (and on) an academic network where they used
several source-fingerprinting techniques to ensure that only properly registered machines (that had been redirected on the first attempt to access a web-page to a server explicitly for registration) were given full(-ish) reign to do their network chattering. But I digress, and they never let me see the
entire database that was produced, either.