For years I've been accessing Webmail through Firefox on my mobile phone (Android in the past and now iPhone 12 mini) and it has worked well: 

https://webmail.shaw.ca

It's a mobile-friendly version which is different from the desktop interface - fewer emails visible, larger buttons, etc. In early August it changed so that the desktop version loads - many rows of emails and tiny text -  really hard to use on a small screen mobile.

I just checked and the mobile version still seems to load from Safari on the iPhone.

Any idea what changed ?

I've had the same @shaw.ca email address for a long time and that's what keeps me with Shaw (and now Rogers). If it stops working I could be switching providers. I don't want to use a mail client on the phone because I don't want to store all my emails there. The Webmail client doesn't take up space - it's just a Web page.