IPhone, Apple id & password changed on old iphone & windows pc; need to link iPad to those changes.

i had problems with my apple id because it wouldn’t sync with my new password. I fixed it on my windows pc via help here, and now can log in to my new id & pw on windows pc. Then i left my phone in the rain (unfixable) and I got a new used iphone 6+ Which works fine w/my icloud id & pw. I had not used my ipad in months (like 8), and i just turned it on and updated to ios 11.2.6. I find i cannot log in because i no longer have any idea what the pw is and the id is not the same as my phone. What i want to do is, change my id on my iPad to the icloud id on my iphone, and then change the pw to the one on my phone too (without having the proper id credentials). ... this really IS my iPad! Also, my ipad uses my old iphone 4 digit number and i would like it to use the same 5 digit number to open as my phone does.

Thanks for any help you can give.