Struggle with NS records (apparently) for subdomain accounts

Hello, a couple of months ago in my work we hired a reseller hosting service that works with cPanel and WHM. This plan, we have hired it to work with a single domain, but still to give independent access Cpanel to many users, always using subdomains of the single domain. This requirement is because, in fact, we are not resellers. I work in a city cultural center, trying to bring the web development to the general public. So we offer practical coursers to begginers. We teach people how to hire a domain and a hosting, why they need them to create and manage a website by his own, what CMS are and how to install them when made with PHP and MySQL... Initially we have been able to do this subdomain accounts without any problem. But for several days, different subdomains have been not working for days and hours, successively (the website and the Cpanel access). One day just a pair of all the subdomains we have. Then the next day, maybe that pair are working again but another one fails, and so on... Of course I have already talked to the hosting provider, but the answer they give me is contradicted by the fact that everything worked properly for several weeks and neither has helped to solve the problem. While I wait for a new answer I am trying to investigate on my own. Aboout their answer: when I create the subdomain accounts from WHM, two name server records (NS type) appear checked by default. This records correspond to my domain, like ns1.mydomain.tld and ns2.mydomain.tld. Well, the hosting provider told me the problem was actually that thes two NS records did not exist. They told me taht everything would we solved simply by creating them myself, as an A record type (pointing to the IP of my reseller plan) from the DNS zone in the cPanel acces for the main domain (mydomain.tld). I did it and also waited several days for the propagation of the DNS but the problem persists. As I said, I have returned to claim my hosting provider, but before he answer me, I would get more arguments to refute if they tell me again something totally contradictory and if the seeks to avoid any responsibility on their part. I've been looking at this forum and I've seen enough people used to create separated cPanel accounts for subdomains. However I have not found messages that recount similar problems to mine to keep such accounts. Therefore, the issue problably has nothing to do with either cPanel or WHM, but still, I ask you here to have more arguments to dismiss it and because I know that many forum users are a lot more experienced and savyy than me. Who knows if anyone has found anytime with a similar problem and then can give me some useful clue although the problem does not come from Cpanel nor WHM. Thanks in advance for your help (and sorry for my poor english).