How do I stop Yealink T28 remote extensions de-registering and staying Not Registered every day

I have two off-site extensions both with different ISP’s on different sites. Both register and work fine until they decide at non specific times to de-register and stay that way until reset. I’m using PIAF 1.4 (elastix does the same) and FreePBX The phones are both Yealink T28 running software. I can even see it starting to de-register as the extension icon goes grey about 15 minutes before it goes completely grayed out
Does anyone have any suggestions?