Did you perform a factory reset to default (clear nvram) when upgrading to 108? Internet should be working concurrently with M1 voice with HG256 connected to port 3 of the N66 with baked in M1-Fiber settings under the WAN Advanced Settings. Just make sure you also power cycle both the ONT and the HG256 when you reassign and reattach the ethernet cables.
When I first upgraded to 102, I did do a factory reset and power cycled ONT, HG and N66 but it still wouldn't work. I did not do a reset for 108. As an aside, I would find it rather user unfriendly for a hardware implementation to have to do a factory reset each time there is a firmware upgrade. Regardless, if I have the opportunity, I might try to do that over the weekend.
It's good to know that your N66 works with M1 voice. In my earlier postings in this thread, I have screen shots of my settings. Could you please take a look to see if there is something I did incorrectly? Alternatively, could you post your screen shots so that I can duplicate them. Thanks.