MTN does not inform clients that dual sim is NOT dual sim

Why must I ASK or INFORM MTN that I want a dual sim phone when upgrading, when the phone I want IS a dual sim. Must I inform them of all the specifications of the specific phone? What if I upgraded over the website? Nowhere it states that the 2ed sim slot on the dual sim phones supp**** by MTN are disabled. The previous phone I took on MTN was dual sim, WHY would I think of ASKING them first whether the phone I'm upgrading to (which IS a dual sim, the Huawei P20 light) is a dual sim. I specifically chose a phone with a large memory because I need both sim slots for my two sim cards. Even when I phoned their customer care they said the dual sim must work becuase the P20 light is a dual sim. Now I sit with dual sim phone of which I can use only of my sim cards, and with a contract with only 350MB of data, because my second sim was my data sim. They suggest I must carry around a router! Will I EVER have NOT problems with MTN when taking out a contract?!