What data allowance is your mobile contract, and are you currently using the router provided by the mobile network operator?
If you are just tethering to your phone and there is anyone else living with you that when your phone goes out with you so does the internet. Including the "smart" of a smart TV. Also any internet-related security camera link.
Even if you live alone tethering could quickly wreck your battery.
When you say your phone and BB are with one ISP, I read that as your landline phone/broadband. However you may mean your mobile phone is with the landline provider.
As one of the first on these forums to ditch the landline completely four years ago, it was clear to me that it was imperative if I wanted to save money over the first 12 months, (my prime reason), to do the deed ASAP. Keep that running while you try to improve the mobile broadband means your "bang for buck" is gone! A wasted expenditure once you have decided the mobile broadband works reliably enough.
Like you I started in December. Stopped using the landline broadband immediately and a month's notice on that given a few days after Christmas but before New Year, having waited to see the effect on mobile from all the new Christmas present phones, tablets, consoles etc. being turned on. (I could do that as there was no charge for ending the BB and no charge for starting it again if I changed my mind).
The landline was a separate contract at £14pm and a month's notice given on that early January. Again no charge for termination.
Come the end of July I shall probably replace my mobile router with the 5G one from Three now that decent 5G is available on the upstairs window ledge that it sits on. (Contract term ends then). 5G doesn't work downstairs at all. Three is the only 5G available in the whole area.
Connections: OnePlus 8 Pro on Three 4+ (LTE)/5G and at home Three Mobile, with (Three)ZTE MF286D router giving about 113/20Mbps.
The best of all possible countries.