If your niece is getting better service in the same locations as you, you could compare why, and adjust accordingly. Switching providers may not help, as many towers are co-located.
Is your niece on Telus or a different carrier? If she is also on Telus then the issue is your phone.
When you switched SIM cards did you get a new SIM or just tested with someone else's SIM? Try swapping SIM cards with your niece and see if the service quality is associated with the phones or the SIMs.
You say you have 3 phones. Do they all have poor service?