Digital tv requires a certain signal level (threshold) before the tuner locks, usually above 6db, a good tuner will be less than this. The tuner is built in to the tv or set top box, so it is worth doing your homework and finding the tuner type used in the equipment before you buy. Changing the receiver might help.
The old analogue signal would deliver a picture (even if it was bad one) with less signal.
The newer aerials have advanced to combat the problems that have been created by giving part of this broadcast spectrum to 4G, also they are better designed and manufactured.
You don't give your location. Check for your nearest transmitter at
Saorview | Coverage Checker. This will tell you the frequency, polarisation and direction to the nearest transmitter.
Going from the top down, normally, you will need a full band or band C/D, directional aerial, vertically polarised (this means that the elements i.e. bars or Xs in the aerial will be up/down) look for one with high gain.
The flat panel digital aerials are low gain and not very directional, don't know the model number of your Triax but last time I looked they were low to medium gain.
Connect it to a masthead amplifier. Then join it to your aerial feed cable with a good combiner/ splitter like the Antiference 75 series.
Where the feed cable joins the rest of the house, add the power supply for the masthead amp and an in-line LTE filter to combat 4G interference, but better to get aerials (one for vertical and one for horizontal) with the filter built in (fewer connections) and you can do without this, if the Triax is old, it won't have this.
Then you can add your distribution amp or splitter.
The masthead amp is better than an amp in the loft or near the set, as having the amp any distance from the aerial will amplify interference picked up in the cable and connections, you need to amplify the signal not the noise.
Try to connect cables and boxes with as few connections as possible, each connection can lose 3db of signal (the signal strength is halved), even if this means losing the convenience of a wall plate connector for your set top box and having the aerial cable long enough to pass through a hole in a blanking plate on the wall (old style) into your receiver.
