i just wanted to make sure it wasn’t the custom install that i had found which might help. i found a few different posts in the linux mint forum about issues with realtek wifi drivers. the most comprehensive one shows issues going back a few years, but this page has the most up to date answer. the stated reason in that post is that there is some issue with the installed driver and 4.15.* series kernels (going back to 4.2 according to the repo readme).
i didn’t ask what kernel you are using, but it will probably be helpful to check with
uname -a. my recent install of 18.3 (along with recent update && upgrade) has a 4.15.0-54 kernel so i figure yours might be similar. the solution in that post directs you to clone a repo with “the newest Realtek rtlwifi codes”. the readme of that repo says it won’t do you any good if you have a 4.17 kernel or newer so checking yours will let you know if that might be a fix you want to try.
if you decide that is a fix you do want to try, i would suggest creating a system backup just in case. the lure of fixed wifi is strong, but at least at this point you have wifi. being able to back up to a partial working system is better than being stuck with an entirely broken one in case this fix doesn’t work on your particular system.
to that end, i have found a few different places this solution is suggested. this is another. both are by the same person who not only has many posts, but also a high level which i assume means they have some respect in the community. add to that the fact that there are replies on both about how the solution worked and i feel like it might be a good one to check out.
finally (i know this is getting long) if you make it through the solution from the first link and it works, there is a bit of extra at the end of the second one to make sure your wifi antenna is set correctly.