Home routing/networking question

Sorry, this will be a long description with a whole bunch of questions peppered through. Thank you in advance for your time and any answers.

I have recently moved in an apartment building that provides internet access to its residents through agreement with a local ISP. There is no modem, just an Ethernet port in the wall.  The problem is that I am unable to connect my router to this Ethernet port as I had done in the past (WAN port on the router goes to live internet connection, whether it be cable or dsl modem). When I do that the router gets no ip address from the ISP (ip is shown as I have tried a variety of troubleshooting up to factory resets of the router, mac cloning of various pcs, making sure cables are ok, etc. No matter what I have tried I cannot get the router to get a public ip and provide NATted ips to the other hosts.
What does work, is connecting the wall Ethernet to any of the LAN ports on the router. This bypasses the routing functionality of the router and allows my devices to connect. Although this accomplishes the main goal - somewhat (see 4 below) - it creates some issues and questions for me.
1. all hosts on my network now get public ip addresses rather than  private ones. I view this as a security risk for my computers, not so much for my other devices. I would like to know what suggestions people might have for mitigating this?
2. The router gets a dynamically assigned ip address which I can't access because I don't know. To access it I have to give it a static, which is all good, except with this setup I am losing some of the visibility of the attached devices - I am seeing unknown "wired" devices which I assume are the ISP's equipment. But I am concerned in the future I may be missing intruders because...
3. To me this setup looks as if I together with everyone else in this building are part of a nice large LAN, with nothing to separate us. Indeed, a quick IP scan found some 60 active ip addresses in my subnet...I didn't try accessing any though they respond to pings. Which raises the possibility for me that someone else might do the same thing I did but not stop at scanning the ip...
4. About the 'somewhat': my Chromecast is behaving very odd under this new setup.  I can control the device, but after the initial set up, and casting, it has stopped  working. Though I can still control it...

In addition to the questions and concerns I mentioned earlier, I would also like to know:
- is there anything anyone could possibly suggest that would resolve the original issue,  namely allowing my router to act as a router/dhcp server?
- is this set up that I stumbled upon inherently less secure - as it seems to me - than the standard NAT-based one?
- and if so... and if there is no way to avoid it here... what remedies can I try?
-to make things worse, the current ISP is the only one allowed in this building. Of course I have contacted the ISP, and they played the fools, along the regular line - that they don't support routers, and if the connection is active and it works their job is done etc...
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When you plug into the wan port of the router, is the router assigning local ip to devices. If it is can you access the router interface?
castalianAuthor Commented:
Yes, it does assign ip addresses, if I leave dhcp turned on.
And yes I can access the router.
Have you tried Cloning the mac address of the router to the same mac address of one of your devices that does get an ip when connected and see if the router gets a public ip address?
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castalianAuthor Commented:
Yes. I have tried using the mac address of two computers, both of which connect fine when connected straight to the wall. Router still didn't get a public ip. I should add that the internet light on the front of the router doesn't even light up. However, the router is a known good router. As in: it was working three days ago at a different location, with a different ISP, And, upon discovering the problem here I took it back to the other location and it still worked fine.
JohnBusiness Consultant (Owner)Commented:
Can you try a different router. You should be able to connect the WAN port of the router to the wall socket. Set the WAN port to DHCP and subnet mask of Perhaps ask someone if they know what the gateway address is.

If your router does not work, a different one might work.

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castalianAuthor Commented:
Another brand router turns out to work just as expected, although the original router also works fine at different locations
JohnBusiness Consultant (Owner)Commented:
So then it is a router issue (which can be location specific because of other equipment) and that's your answer here.
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