You need to contact 48 support.
See the link in my signature below.
Some people had to get a new SIM to get coverage after the 4G change.
But then they had no coverage at all and not just poor internet speeds.
Also make sure that your internet settings are correct and maybe try deleting you existing ones and configuring them from scratch:
https://community.48months.ie/t5/The-Knowledgebase/My-Internet-is-not-working/ta-p/45928Hope this helps.