I also tested connecting another domain, but the site is still not available

You need to add both @ and www CNAME records if you're binding your root domain to a CNAME record.

maybe add dns record for your ipv4 addres and wait then perhaps all the dns records will automatically be set for you ipv4 addres and they will all function under one maybe that's the case with your records if the records "later on" shows to be functioning with one and the same record, depends on how you configurate the domain after following with the records and so on, if they later proffes to be functioning under one the domain automaticlly enters the same dns record for all the implicated records

You should not be adding A records for a custom domain for Railway.

Railway only provides CNAME records.

I added all the necessary entries and did it correctly! I'm sure about it because this is not my first time working, and everything worked!

I know this. I'm adding everything correctly for each of my sites! This isn't the first time I've worked with this.

It's just that at some point, the server and domain connection went missing. I tried to make the connection again, but nothing changed

Try using Cloudflare's DNS then...

Not exactly sure what the issue is then if everything was added correctly.

I don't really understand how I can add my domain that I bought on netlify to Cloudflare if I can't change the NS records.

and I need my own domains that I bought and linked, not any other ones :(

If you're not able to manage your DNS in Netlify for some reason and want to manage DNS in Cloudflare, you'll need to transfer your domains out of Netlify. Instructions for doing so cab be found on their website:

https://docs.netlify.com/manage/domains/manage-domains/transfer-a-domain/#transfer-domains-between-registrars