How to Be Firm with Vendors Without Coming Off as Nasty?

I've always tried to be friendly and accommodating when dealing with vendors, but recently it's backfired on me a couple of times. In one situation, I reached out to our Verizon rep about setting a certain DNS for four iPads we use because our app doesn't work with the default Verizon 5G settings. The rep was supposed to check back with their engineering team, but after weeks of waiting and my follow-up emails—which were ignored—I'm really feeling the heat from the iPad users who can't access their essential app. They've suggested I bombard the rep with daily emails until we get an answer, but that feels excessive and mean to me. In another instance, we replaced an old software with a new one that has been nothing but problematic. Each fix has led to more bugs that make it even harder for our team to work. Users got so frustrated they emailed the vendor directly, and unfortunately, twisted my words in the process. I understand their frustration, but I really want to maintain a good relationship with the vendor while still getting the issues resolved. People have told me I need to be more assertive, but I don't want to come across as rude or aggressive. How can I push for better responses without being seen as nasty or confrontational?