Why is my phone intermittently getting no signal?

So I have a samsung s9+ which has recently developed a problem connecting to 4G. I replaced the charging port and battery and then 2 days later it started getting problems connecting to 4G. It will work for an hour or 2 then all of a sudden won’t connect. Sometimes it loses signal all together and has a circle with a / through it and sometimes it says it has signal but doesn’t have the 4G symbol as normal. Does anyone know what I should be looking at to fix it?