Hi Guys,
I run in to this problem too with my old E122 locked to Three Ireland (doesn't really matter which carrier it is locked and actually it doesn't really matter if it's locked or not);
There's a very nice, official software on Huawei website that is not well advertised for obvious reasons (Huawei makes contracts with mobile carriers so it would be against a good partnership; still, it's there and we should use it - that's my opinion 😉)
To cut it short - tested on El Capitan 10.11.4 - works like a charm even with different APN profiles etc.
Go to: http://m.huawei.com/enmobile/consumer/support/ (or just navigate to global Huawei website and ensure you're in Consumer section, then go to support).
Find Mobile Partner for Mac 10.11 and download. It's a very good software, mobile carrier independent that allows you to create multiple profiles (if you know the relevant APNs) and use different SIMs from different cellular networks if your modem is unlocked;
What's best - it picks up default APN settings over the air so probably most of you will have it working "off the box".
My example: E122 bought locked to Three/3 IE about 7 years ago, I was about to throw away; decided to try to unlock it and did it today; couldn't make it to work with my Vodafone SIM but the blinking LED was showing blue which meant it picked up proper network; I tried different ways, even the one from this thread with no success, until I went to Huawei and picked this Mobile Partner; it connected straight away even if I didn't put any Vodafone settings - it just worked 🙂
I tested that with second, private APN and it flies like a charm.
Hope it helps.
[Oh! It doesn't really matter which model you choose, select something similar to your dongle - mobile partner is just a generic software so it works with different devices. My E122 wasn't anywhere on Huawei website...it is so old 😀)