@qmaness You are setting up a configuration of MiaB that is not well documented in the guides how to actually do IMHO.
You have chosen to use “External DNS”, maybe not consciously, but your scenario and actions show that you have made this choice. When using External DNS, it is not necessary to create glue records, so your ISP was actually correct but they seemingly did not explain this to you in a correct manner.
Now let me explain what you are going to need to do. You actually have two options when using MiaB for your mail server and a different server for your web server. These two options are especially relevant when you wish to use a different providers DNS service - by different provider, I mean not using MiaB for DNS.
Option 1 is to use a ‘service domain’. By ‘service domain’ I am referring to a completely unrelated domain for the specific purpose of operating your MiaB server. The downside is that there is an extra domain to maintain and the annual costs of that, and the upside is that it is not tied to your actual domain at all, so there is no need to be concerned with modifying (or adding) glue records for your website domain, or even pointing DNS to your MiaB.
Option 2 is to use your domain. When using option 2 there are decisions to be made before you begin such as do I use the MiaB for DNS, or do I use external DNS? If I use MiaB for DNS how do I avoid a ‘single point of failure’ and how do I set up Secondary DNS to avoid that? Factors such as will there be a web site hosted elsewhere need to be considered. So you have to put a lot of thought in to how things will be accomplished - whereas using a service domain, nothing has to be pre-planned.
Now @qmaness it is here that you have to make a choice. Do you purchase a second domain and follow option 1, or do you continue to use a single domain and need to follow option 2.
I am going to make the assumption of option 2 here … since that is what you are asking about in your OP. And since that is the path you have ventured down. One of the decisions that you have to make is whether or not to use MiaB for DNS or External DNS. You have already chosen to use External DNS. Your External DNS provider is Linode. I am going to validate that you have made the correct decision to use External DNS IMHO as you are running a commercial website on your domain. Those running private or hobby websites are not counting on their domain / website being absolutely available 100% of the time so using MiaB and the single point of failure it introduces is not a critical concern for them.
So when using External DNS with your MiaB on your own domain, you skip the set up instructions to create glue records. You will need to have the name servers set by your domain registrar as being those of the External DNS provider - in your case Linode so ns1 to ns5 .linode.com need to be set.
This step is optional, but is a good practice - especially for someone not well versed in how DNS works, so I recommend not skipping it: After your installation of MiaB is complete, you will need to go to the “Custom DNS” page in the admin area and add the A and AAAA records for your website. In your case, you will have to add 4 records - the record for box will already be correct. Then once that is complete, since you are using External DNS, you need to go to the External DNS page and copy ALL records on this page to your DNS records at your chosen provider EXCEPT for the NS records as those will be handled automatically by your DNS provider 99% of the time.
This will be the correct way to set up MiaB using External DNS and hosting domain websites elsewhere.
And I realize that I lied … solving the OP’s problem will be in the next reply as this one was just a description of the two different ways of setting this up.
So, on to how to fix the mess you have @qmaness …