If memory serves, tethering was more of an issue back in the mid-2010s when 4G was relatively new. EE were fine about it but the other providers got shirty. In the 2020s, I've tethered fine off Three, BT and Plusnet and just be mindful of the mobile data allowance that you get given per month. This can be monitored in system settings of the device that you're tethering off - set a lower value as a warning, then an upper value (the monthly data allowance) as a final warning. i.e. I pay £8/month on Three for unlimited minutes/texts and 25GB data. I set my first warning at 23GB and my final warning at 25GB as then I know I'll incur extra costs if I go above the 25GB.