Huawei reading my Office365 email?

I have recently moved into rural broadband area, so received a new Huawei B315s modem from Wireless Nation. With an external aerial we get reasonable performance (30/10), but about 2 weeks after installing we all started getting some odd messages:

> "Please reset your gmail passwords" - appeared to all users using either a private gmail domain (richard@xxx.co.nz) or abc@gmail.com. Applies to all devices - IOS, windows 10, android. The dialogue box is a valid email client dialogue.

> Security alerts: "outlook.office365.com - information you exchange with this site cannot be viewed...however there is a problem with the site's security certificate". The security certificate associated with this dialogue is called mobile.wifi and is registered by Huawei in China.... - snapshot of the error here:

> a similar alert occurred on our aotudiscover record as well.

Not being a security speicalist I did some digging and wonder if the modem is implementing a proxy in the network, and so effectively performing a man in the middle attack, routing my email via their servers?

So I change the network - implemented my Fritzbox as the main router and DNS server, using the Huawei just as a gateway, but on the same LAN segment (I could not easily setup a DMZ as I use the Fritz for VoIP). Unfortunately the same recurred - and when the error hits the PC slows down to almost stationery. I reject the certificate and in many cases this causes the modem to go into a spate of intermitent internet disconnections.....

So to my question:

Firstly has anyone else come across this?

Secondly am I correct in my understanding - and if so are Vodafone / Spark / Wireless Nation aware of this? Is there a secure solution?

Thanks

Richard