Your parents might consider Starlink. My mother is in a similar situation. Way out in the countryside with only very slow DSL available. Even the two WISP (Wireless ISP) providers did not have towers in line-of-sight of her property (darn trees).
She got Starlink and now is getting about 80Mbps down and 40Mbps up.
It is a bit pricey at $550 for the unit delivered and $99 a month. But when you don't have any other realistic options...