E-mail: firstname.lastname@example.org + my PGP key (old one)
You can also use my ProtonMail address: email@example.com
I also hang out on IRC (mostly Libera.chat...RIP Freenode) under the nickname lexik.
If we know each other, then you probably have my phone number and you can also reach me securely on Signal.
Please note, that phone eavesdropping is a common practice in Czech Republic. Consider other, more private ways of contacting me, before dialing my number.