Modem issues kept going on.
Started complaining to them again and requesting they send one of their proper modems.
Service: "Unfortunately it is not possible to request a different modem"
Me (tho *slightly* more polite): "I don't care, you guys caused this issue by sending us a shitty modem, so you guys are now to send me a proper modem, period. I'm done with having unstable internet because <ISP> doesn't want to admit their modems are flawed by design and deliver proper ones."
Service: "I have good news! We can swap out your modem for the one you requested"

Sometimes, you just have to bitch hard enough.