If you only get apps from the play store and you have a password set on your phone, then you should be relatively secure. If you download apps from any janky website then no anti-virus is going to protect you. However, what most people think as a virus or malware is actually their credentials being compromised. Either shared with someone or cracked because the user name and password are easy to guess, or god forbid, have never been changed from the default.
A true virus or malware is usually to steal data and is designed to be undetected. If weird things are happening it's probably not an infection.
A server error would only affect a specific connection to a database and wouldn't be a connectivity issue like a bad wifi password. Captive portal is a way shared network connections require a login from a web page to access the internet, like when you stay at a hotel of go to a coffee shop that requires you to accept terms of service when using their wifi.
Can you be more specific about the kinds of things you see changing?