you got problem with the modem ? You do not have money ? even if you change your modem or router you still need to make sure your new modem ok.
and actually you can change your modem with your isp but what if your isp modem just suck, what you need to do?
buy your own modem no need to save your money, what modem or modem would i suggest? i would suggest Mikrotik Modem to you.
why? because u not just can get very stable modem you but also you got variable function in the box and you do not to very to configure it because it got very huge mikrotik configuration database which the url is here