phd - Is it unethical to enroll in a low-ranked institution with the intention of switching to a better one if possible?

I am an international student who has been admitted at a low ranked institute in the US. I also have an offer from an institute in my own country to do a Masters'. I ultimately want to get admitted at a prestigious program in the US.

I was recently advised to join the low ranked US institute, do my course work well, establish contacts with professors at more prestigious schools, and then switch to a better school for a PhD. It sounds immoral to me. But I was told it is not immoral in practice as the institute receives thousands of offers to fill up the seat you will be vacating.

Is doing the above seriously unethical?