Does "tried this" refer to ideas further back in this thread?
Read this below. It might help. If it doesn't, it may be a hardware issue, requiring replacement of the Wifi/GPS chipset and/or antenna.
On the iPhone X, my issue was (TTFF) time to first fix i.e. getting the initial GPS-based location. Once I got that, the locations were always accurate, if I started my journey without waiting for that, I had problems.
TTFF seems much better now probably because I did the following:
SIM trick (see previous postings and warnings about removing/reinserting SIM cards while phone on)
Set Compass to True North
upgraded to iOS 12.1
turned off all GPS apps apart from the one I am using
turned off all other unnecessary background apps
stood motionless in clear view of sky until I got GPS-based (not Wifi/Cellular-only) location. to be absolutely sure of this, I use Motion X and look for the blue/green globe to spin, and sometimes also use GPS Diagnostic.
All of these may have helped, particularly the first, third, and last steps.
Even after all that, TTFF is still slower than on my wife's iPhone 6S which uses the same carrier and same iOS.
Anyway, apart from looking like an idiot standing on the spot outside for 10 to 30 seconds to get the first fix, I have no problems at the moment.