I am new to this so apologies if this is the wrong place for such a question.
I'm testing out a simple "hello world" website hosted at home. As we have not been assigned a static IP address by our ISP I've set up a subdomain with a dynamic DNS provider to point at the home router - let's call this subdomain mysubdomain.ddnsprovider.net. I also have a domain which I own (and which was set up with a different provider than that to the dynamic DNS subdomain) - let's call that mymaindomain.com. A DNS CNAME record for mymaindomain.com is set up to point at mysubdomain.ddnsprovider.net.
I can successfully browse to mymaindomain.com and see my "hello world" page from anywhere outside of my home LAN - but if I browse to mymaindomain.com from any machine at home (i.e. within the LAN) I get "Server not found"/"web page not available" etc errors.
I understand from various searches that this looks like a NAT loopback issue and as such is likely best solved by getting a new router that supports NAT loopback. However, I can see my "hello world" site if I browse to mysubdomain.ddnsprovider.net from anywhere at all - including from within my LAN. From what (little) I understand about NAT loopback, I shouldn't be able to connect to a site hosted within my LAN via an external address (i.e. mysubdomain.ddnsprovider.net) at all if lack of support for NAT loopback by my router is the cause of the issue.
So, before I go out and buy a new router, my question is:
Is this definitely a NAT loopback issue (how can I confirm that)? And if so, what's the reason I can see my website from within the LAN via mysubdomain.ddnsprovider.net? If it isn't NAT loopback what is it likely to be and what's the best solution?
FWIW - I'm in the UK and am using a BT home hub 3 wireless router connected to a BT Openreach modem (Infinity fibre). Does anyone know whether the lack of NAT loopback support is inherent in the router, or modem, or both? - i.e. is replacing just the router likely to solve the problem or is it likely to persist unless I replace both of them (if that's possible).
Any light that anyone can shed on this and any advice would be very gratefully received.