Personally I like ModemManager because of its features and design. On the other hand it's a massive piece of software with all the dependencies compared to the usual OpenWrt minimalism. I suggest you test both, and also the packages from @IceG as they are not compatible with ModemManager.
I haven't had any disconnections, but since my router will be at a remote site, I have configure watchcat to reboot it if connectivity is lost because there could be many reasons for the loss of connectivity and I wouldn't be able to troubleshoot.
For ModemManager reconnect have a look at: