So does that ONT belong to you then or will you have to pass it onto the person you sell your place to?
The ONT you just leave it there. If the next person want fibre they will likely swap the ONT or maybe just activate a line for the new tenant on the same ONT. You are not allowed to take it with you. The thing is 100% useless and can not be used a new property anyways so yeah just leave it.
The router you can take and return to ISP if it's a loan unit or keep it if they gave it to you after a duration.