@fallafalla, Do you have the config in hand or how to guide? Thanks
Sure thing one caveat is that automatic updates to the VoIP settings on the router will no longer occur requiring a temporary re-connection directly to the internet, at least this has been my experience though it appears to be fairly infrequent anyway. This last update being the only one this year that's caused issues.
Now I'm no network engineer but this is working for me so:
Step 1 Connect Slingshot router to ONT/Internet directly to allow remote management to set the parameters on your device (this may require a factory reset). Confirm phone is working at this point.
Step 2 Log into Slingshot router and change settings of the Lan DHCP server to be outside your normal network I.E 192.168.50.1 to avoid any potential conflicts.
Step 3 Reconnect routers as follows : ONT -> GoodRouter wan port. GoodRouter any lan port -> Slingshot Wan Port
As you already have a third party router working I'll skip that part and step 4 assumes you have an internet connection to your GoodRouter, if you need any help with that shoot me a PM.
Step 4 Log into Slingshot router using the 192.168.50.1 set earlier, go to basic setup, select ethernet wan, next page select IPoE, next page select no vlan tag, next page select obtain IP automatically, then apply/save.
At this point the Slingshot router should obtain an IP from your GoodRouter connect to the internet and register itself with the VoIP server. This may be more detail than you need (no idea of your familiarity with networking) but may help someone else in the future. I have also left out how to get the third party router working with Slingshot due to the differences in a PPPoE vs IPoE setup.
It also wouldn't be a bad idea to add a static IP for the Slingshot router in your GoodRouter though not strictly necessary.
If you have any issues I can add a picture or two of how the config for the Slingshot router should look all said and done, hope this helps at any rate.