mygamertagsucks

No worries, that's exactly what this kind of support is for so you can answer when it is convenient for you, we'll keep answering if you do. What do you mean by "picture of the speed test run from my cellphone, after being able to get online via Ethernet port, but not via WiFi" are you saying you have an ethernet adapter for your phone that you were using?
The firmware doesn't go back if you factory reset, it will stay on the one you're on but you won't be able to miss that you've factory reset correctly if you see the setup wizard instead of the normal interface. Okay keep us posted!

General errors like that I don't always trust because all it's really telling you is it wasn't able to connect and there could be another reason for it BUT If you're getting a DNS issue then that is a problem, basically DNS is what turns a URL for example netduma.com into the IP address that you use to connect to that website. So if that's not working properly then it means that entering google.com into your web browser for example it wouldn't know what IP to connect to/access to then display the correct information back to you. If that is the case the ISP should be able to sort it quite easily, either from a new modem (if this is the issue then wiring is less likely and now more likely the modem) or something they can sort remotely. 
I understand your frustration on that, they have their script they have to go through and they will go through it regardless of what you say. Your best bet is to go through the motions, tell them the issue, do/feign doing if you've done their suggestions before (make sure to leave enough time in between so they can be certain you've done it) and let them go through all the steps they're required to ask you. If they can't fix it with those basic suggestions which is likely then they will either want to send a tech out which as you've mentioned isn't viable, hopefully explaining that they will (or ask to be) escalate(d) to a tier 2 or 3 tech. They will have more knowledge and should hopefully provide more targeted support/be able to identify the cause and solution. Also absolutely do not mention another router - it's vital you're clear that this issue is experienced when directly connected via ethernet from your PC to their modem - not saying they will but some may use that as the excuse to get you off the call if they don't know what the problem is. 
Good luck, I hope you can get it sorted and would be interested to hear an update on how you get on!

@DanologyUKThat tool isn't generally reliable for identifying the source of an issue, but it's great for 24/7 monitoring but it is free though  @mygamertagsucks As he mentioned, the specs fall within 3 dBmV and 40 dB. The only potential concern might be the upstream, but it's still acceptable. It's likely difficult to fine-tune, and the ISP probably won't invest resources for such a minor improvement anyway.
So did you get the ISP to check and are you still experiencing connection issues? I would more or less recommend PingPlotter for in-depth diagnostics. The only con is it is not free but there's a trial. The best route is a perpetual license version. I got Pro, but it is because I am pretty deep into analysis.

It's always a good idea to get clarity! Good advice above but that could be it. Fingers crossed they'll be able to sort it out for you. Do keep us updated!

@DanologyUKThank you. I honestly figured this was the case, but was hoping y'all would tell me otherwise. I like the analogy, btw. And, honestly, when you said I was worrying over nothing; you have absolutely no clue how square you hit the nail on the head with that, lol.

As above the DoS attacks are rarely attacks and are usually showing connections you've made, for example some of IPs from your logs are:
3.169.149.83 - Amazon
69.147.65.251 - Yahoo
142.251.41.78 - Google
185.146.173.20 - Shopify/Cloudflare
100.52.165.89 - Amazon
The others look to be reputable as far as I can see. There is something that potentially by the looks of the MAC addresses and the fact that they don't show as legitimate device on lookups you can solve which may help, is that you're likely using iPhone(s)/Android(s)/Windows PC(s) via WiFi that are using Private/Randomized MAC addresses which would explain the constant reporting of them in the logs. If you disable this option on each of those devices it should remove the entries from the logs and hopefully if that's the cause stop the issue you're having. If not then I would suggest connecting a PC directly to your modem and see if you get the same issue, if you do then that's something you'll need to contact your ISP about.

Dos Attacks seen in the logs are NOT DDoS Attacks. If you search on the forum, such as this post, you will see they are not DDoS attacks 
 

You're right. My bad. I just thought that the XR500 bricking issue was kind of a known thing. At any rate, thank you for your support over the years. I've read a lot of stuff you've written here and there, all of it is top notch stuff.