This might sound a bit weird, but bear with me.
My husband bought Grounded so we could play multiplayer. He installed it on his brand-new, custom-built PC. No matter what we did, he couldn't see my lobby to join, and couldn't get host at all (just got the spinning loading screen, or, maybe 5-10 minutes later, an error/crash). The last-ditch effort was to install the game on his older gaming laptop and...it worked perfectly! The computer it was running on was the only different factor -- otherwise, we were in the same house, using the same high-speed internet, both via Steam, etc.
His new PC is, on paper, more powerful with better specs than his gaming laptop, so it's weird that multiplayer works on one and not the other, right? It made me wonder if the multiplayer connection issue is some hardware or software compatibility problem (?). Obviously it could be a million things (I'm not a developer lol) but I'm curious to see if there's any link to PC specs...
PC (multiplayer didn't work): AMD Ryzen 2700, GeForce RTX 2080, Windows 10, 32GB RAM
Laptop (multiplayer worked): Intel i7, GeForce GTX 1070, Windows 10, 16GB RAM (< my laptop's the same, and multiplayer has always worked for me)