I was having this same issue and I believe I found a solution. Go to settings -> connections -> wifi -> advanced, and turn off "switch to mobile data"
Seems to have fixed the issue for me - hope it stays fixed!
I found this in another forum and it seems to have corrected the issue.
"Type *#9900# in your keypad scroll down to WLAN FIRMWARE DUMP MODE and turn it off than click on WLAN ADVANCED DEBUG and click on DISCONNECTION SYSDUMP and press OK.
When you did this scroll up to DELETE DUMPSTATE and press ok. Then click on exit at the bottom.
When you done all of this type *#2663# in your Keypad and press on REFRESH WIFI VERSION."
I've tried everything as well. Very concerned that Samsung has not stepped up to resolve this. May point to some serious issue that they cannot disclose and thus stay silent. It may be time to turn to Twitter and other social media to shame them into ownership and/or disclosure. For them not to address seems to imply bigger issues in play and this, of course, raises even greater concern!
I've had this issue on and off for at least a year now. Tonight it has been insanely bad. Glad to see it's still going strong with no actual fix in sight. Didn't we just do an update. Actually 2 within the last month or so alone. Nevermind that I have 250mb internet and can't even stream services I pay for (which I used to just fine mirrored on my '15 uhd Samsung TV) not cut out on phone calls even in wifi calling even though my tv can stream 4k and hdr and my ps4pro picks up 100% signal strength always, in the same small room. Also someone else in the basement on the same network with their older iPhone never has an issue. I've always loved my Samsung phones but this has been a catastrophe, and the fact it has never been addressed after all the BS I went through thanks to the NOTE catching on fire. I've tried multiple fixes but there seems to be some new ones. Come on Samsung I've sunk probably 10 grand into your products over the last few years between laptops phones TVs and accessories. How about some help