Have I done something stupid? 😬

Last week I changed our ISP from OneNZ to Contact when OneNZ increased the price of their 300/100 plan to the same as what I could get the 900/500 Contact plan for. So I changed from OneNZ 300/100 to Contact 900/500. I'll admit I never had any issues with our OneNZ broadband, I was just a bit annoyed at the price change. 

Things didn't get off to a great start with Contact when they connected me a week earlier than requested, and on ONT2 rather than ONT1 which meant I then had to notify OneNZ to cancel our plan (with a 30 day notice period!!). 

Anyway once changing the connection over and following contact's instructions for modem settings (from memory no changes were needed) it theoretically is working but the new connection FEELS slower and I'm having disconnection issues.

On speedtest the speeds and latency are as they should be (both on Wifi and cable). 

For wired connections (i.e TV and Desktop) there doesn't seem to be any issues. On my phone (S24 Ultra) and laptop youtube buffers every now and again (never happened previously) and images/videos take longer to load (previously instantly). On my phone the NZHerald app specifically fails to load more often than it does load (stating no internet connection when there clearly is) and images on reddit etc often just won't load. Similar deal with social media apps. It FEELS far worse on my phone than the laptop. My Dyson fan which can be controlled via wifi now has connection issues more often than it's connected, same with my wifi baby monitor. 

I made zero change to my wifi setup when changing ISPs. Literally the only thing I did was plug the ONT1 cable into ONT2. I'm using the OG Vodafone Ultra Hub router, just as a router, and 2x DECO X20s for mesh wifi (in access point mode). 

I've obviously tried turning everything off and on etc - no difference. I wondered whether it was a router issue (it's old, was considering getting a new one anyway) but it feels like it's more of a wifi issue. Like I said, had no issues whatsoever prior to switching ISPs. 

I'm not very techno savvy so hopefully the above makes sense. Any thoughts? Is this just me having done something stupid by switching from an ISP I had no issues with...??!!