If the new router is your WiFi access point that is likely to be the problem. Unless you are actually connected to your access point WiFi geolocation depends on the access point being in Google's database which from past experience can take a few weeks to update. When I moved home WiFi geolocation put me at my old address for about 5 weeks.
Your location of 800 metres away from your true location is the location given by mobile network geolocation using whichever network masts you are in range of. If I switch WiFi and scanning off then restart my device Google Maps locates me to about 600 metres west of my true location. Mobile network geolocation is still accurate enough for services such as weather information as that is area based rather than a specific location whereas smartlock has a range of less than 80 metres.
I would suggest you leave WiFi and scanning on all the time when at home which should resolve it. When your access point is added to the database you should be able to disconnect from WiFi and still have a working smartlock.
It may also be worth keeping GPS on whilst at home, this can help in adding your access point to the database.
There is also a Feedback option in Google Maps which you could try :-
View attachment 124213 View attachment 124214 View attachment 124215
